First, thx.

Then, to answer your questions:

The Zettelkasten was devised to be constantly updated and searched and linked, be it analog or digital. My index is the key entry point, next to the my many bibliographical and authors cards and then I simply follow the links from one card to the other(s).

I can easily update any entry by adding more cards to it, with a sub-digit ID linking it to its parent.

I'm not a huge fan of videos, but you could do worse (much worse) than watch a few on Scott Scheper's channel (https://www.youtube.com/@scottscheper/videos) the only real downside imho, beside the sheer amount of videos, is how much he insists on being hostile to digital. Even though I'm an analog user myself I find his constant attacks tiring (not necessarily all wrong, but tiring) and counter-productive. The same with his book (a really excellent understanding of what an analog Zettel is and how one could use it, with way too much anti-digital remarks). A more neutral and not less interesting reading would be Bob Doto 'A system for writing' (https://bobdoto.computer/) — reading it this very moment and not yet finished, but I like what I've been reading so far. The kind of book I would likely offer to a newcomer.

As for cost. There are many options to reduce it to barely nothing (say, much less than the cost of a cloud subscription to host your files for a few decades):

• You may not use index cards but standard paper. Much cheaper. And it has another advantage: it will eat less space in your boxes since standard paper is thinner than index cards. For years, I was reusing the back of letters and documents, the back of my drafts and so on I cut to size (A6, aka 4x6 in the USA). I made thousands of cards for cheap, if not for free. My only expense was to buy a (used, cheap) office paper cutter (one of those guillotine-like thingy with a large blade that can easily cut 20+ sheets at once). So it was really quick and neat to make a bunch of new 'cards' from my recycled A4 sheets.
• You may buy index cards in bulk or even second hand. I've recently purchased 10.000 A6 (4x6) cards brand new still wrapped in packs of 100 from a shop that was closing down for less than... 40$, shipping included.

For storage, I have yet to find a decent filing cabinet. Living in France, I don't have access to the variety of new cabinets you have in the USA, or only at absurdly expensive prices. And used 4x6 filing cabinets are not that common. So, for the most part I've been using a (high tech) mix of shoe boxes (they work surprisingly well) and of those office index card boxes (plastic or metal boxes with a removable lid and optional separators). Both are ugly as fuck, that's for sure, but at least they do their job. One day, hopefully, I will get my hand on one of those nice and well-made cabinet :p

Edit: typos (part thx to Apple's autocorrect moronic decisions, part because I just suck at English ;)