So my world is a purgatory for dead deities both major and minor. I actually ran a one shot where Saint Nicholas of Myra, the man who inspired the story of Santa Clause appeared as a new minor deity in the world. So I literally have the OG Santa Clause as a conical member of my world.

As for the sleigh and reindeer, they don’t exist, because those were added to the Santa Clause lore in the 19th century. Long after Saint Nicholas. He instead has the ability to manifest a corporeal form of himself wherever in the plane he wishes, visible or invisible. He is also able to manipulate dreams and memories. (I had to give him some god powers.) Everywhere he goes he leaves little trinkets, nothing major or power shifting, but little things that are really useful that everyone needs.

He was introduced as a deity of mundane objects, because he would give gifts that were needed for everyday life but nothing fancy. It took a while for the players to piece together that the deity, who referred to himself as Nicholas, and his followers referred to him as a Saint, was in fact Saint Nick.

You might like "Hogfather" from the Discworld series. Serious. Give it a spin.

My science-fantasy world with intelligent animals living in harmony takes place millions of years after humanity mysteriously disappears from the Earth, I imagine how this goes is someone dug up some ancient human artifact that contained information about Christmas and felt determined to do something similar in their present day (wouldn't be the first time, a lot of their science and technology was borrowed from the humans when their ancestors dug up human ruins and deciphered human language). They might pull off a Santa and sleigh in the following ways:

1. Well all the humans are extinct from the planet and they haven't built Anthropocene Park yet, so best they can do is dress an overweight bonobo or other ape in red and strap a fake beard onto him. Or, if we're using animals that I actually have developed characters for and possibly the much cuter option, Santa Cat!

2. A sack of toys wouldn't be hard to come by, though they would likely have to be species specific since different animals like different things. Not a huge problem all things considered.

3. They have something called a Gravitational Slipstream Hovercraft which are nearly ubiquitous as the primary form of public transportation, think of them like aerial buses. GSS hovercrafts are probably by far the safest way to do this, as they can levitate and fly very fast under their own power, have active stabilization to keep itself steady as a sleigh needs an open top, have automatic collision warning and avoidance systems for both flying animals and other hovercraft, and can lift enough weight to carry a massive bag of toys. They would need to rent a hovercraft from one of the airlines/public transit providers, probably the Feline Ministry of Transportation as the Felines are more or less considered to have the best hovercraft tech. Unless Santa has a pilot certificate and is type rated on that hovercraft though, they would probably also need to "rent" a pilot too. From there, they can build a structure that resembles a sleigh around the hovercraft, which luckily wouldn't be hard to do since GSS hovercrafts don't have wings that need to be kept aerodynamically clear, nor do they have rotors or spinning engines that create airflow. GSS engines can still do their gravitational field manipulation from behind the sleigh facade.

4. They have actual reindeer, who are also sapient just like every other animal, and you could presumably just hire nine of them to pretend to run in front of your hovercraft. As for how they might levitate with the hovercraft while not having anything supporting their hooves, they can use a similar form of technology as a gravitational slipstream drive, albeit several stages more advanced and way more restricted by the government. A quantum interface can levitate pretty much anything within its area of effect. This would probably be the hardest part simply because they need to make a good case to the regulators why they should be allowed to use quantum interfaces in this way, and they would also need a government researcher to ride along with them and to be the actual one operating the quantum interface, possibly even my main character!

5. Rudolph is pretty easy, just strap a red light to her muzzle and pretend to bully her for a bit! The hovercraft has navigation instruments and would be flying in IFR so she doesn't actually need to shine through the fog.

Realistically, instead of delivering presents to individual houses, they would probably just parade around going from city to city and handing out free swag to kits/cubs/etc along the way. I doubt the government security agencies would give them access to their naughty lists so they'll just have to take their word for it on if they were naughty or nice. This world might have its share of Grinches too, mostly in the form of air traffic controllers freaking out and not allowing them to enter their airspace because their aircraft is "unsafe", such Christmas haters amirite?

You might like "Hogfather" from the Discworld series. Serious. Give it a spin.

I've got two answers for this, one for before the apocalypse happened, and one for after.

Before the apocalypse, it's easy enough. I wanted the tech of that era to be near-magical, so that after the apocalypse it's seen as magical.

1- There's already a bunch of mall santas (that profession is still around in the 2100s), just hire one.

2- Get a large sack, buy a bunch of toys off A3 (Amazon-Alphabet-Apple), done.

3- You'd have several choices for this. If you're on a low-gravity world, you can hide propellers or thrusters inside the sleigh and it'll probably be enough to fly. On Earth though, you'll either need to basically build it as a massive 4-rotor drone, carry it with a helicopter, or use anti-grav tech. I'm leaning towards the former, since I haven't decided yet how I want the pre-apocalypse anti-grav tech to be like. That's the flying done, the sleigh shape is just building a decorative frame.

4- Humanity's gene-editing tech isn't good enough to make flying reindeer, so they'll have to be regular reindeer strapped to a rig in front of the sleigh. It might be hard to make that look convincing.

5- Humanity's gene-editing tech is good enough however, to make a reindeer with a florescent nose.

As for difficulty, the hardest part here would be getting permission to modify a reindeer to have a glowing nose. The rest is doable for a small dedicated group.

If they actually try to break into houses though, they'll just get arrested. That is if they don't get shot down first for refusing to identify themselves ("Santa" is not a valid identification number) and going into controlled airspace.

After the apocalypse, it becomes significantly more difficult, for obvious reasons. Note, the post-apocalyptic part takes place on a far-off moon called Eren B, with about 1/3rd Earth gravity and few Earth animals.

1- Finding someone fat might be difficult, as the low gravity kinda stretches everyone out vertically, but it's definitely doable. The beard and red clothes is easy.

2- Easy, sacks are everywhere. Obtaining that many toys would be expensive but that's nothing compared to the next few points.

3- Oh boy, this would be almost impossible. You can "fly" using a Shard Of Gravitation, but the problem is that you only have control over how strong it is, which is why they are primarily used to lighten vehicles instead of fully lifting them off the ground- if you lift off, you have 0 control. There have been experiments with people strapping 5 or 9 Shards to a single platform, with them arranged around the sides, so by changing their strength you can stay upright, but that is insanely expensive- even a single shard is worth more than some villages. And it still only lets you hover, not control your movement. Perhaps, if you manage to find an undamaged fully functional ancient sky-ship (an artefact that would quite possibly be the most valuable thing on the planet), and built a wooden frame around it, you could get a flying sleigh. I suppose another way would be to invent airships. Eren B's low gravity make them a lot easier to build, but that would still require a bunch of technology that just isn't available to them.

4- Impossible again. Reindeer don't exist on Eren B. The original terraforming effort didn't move over the entirety of Earth's ecological diversity, as that would have been pointless. Reindeer were left behind, together with most of the substitutes you could use. The closest you could get would be to strap horns to a horse, but that's still difficult, as there are very few horses on Eren B and the ones that are around are carefully guarded by a (currently unnamed) group in the south, because of the large advantage it gives them. It's not entirely impossible to get a couple from them though, either through trade or by stealing them.

5- I guess you could strap a lantern to a horse's nose? You're already strapping horns to them.

This would be impossibly difficult. No.3 especially is the issue, even if the entire planet was dedicated towards this goal that would still probably be impossible.

Edit: should mention my world is basically cowboys vs indians IN SPACE, and everything is an excuse to have cowboys and indians IN SPACE. Theres culture laws, and the world is a stage play almost. The public schools even teach accents from millenia ago.

The government forms another cultural committee, which in turn creates an new government department. They stage presents at train stations, with significant amounts paid to the railroads. At the end, parents are still in charge of getting them under the christmas tree.

Most people involved are honest, but they fail the audit, and a hundred tons of gold is missing, even though they never needed gold for the project. An inveatigation is opened, turns out it was shipped off world. No one was arrested, but one railroad owner ends up shot by a trigger happy deputy.