Google just has a lot of storage space. They have dozens of data centers, each of which is an entire building dedicated to nothing but storing servers, and they’re constantly adding more servers to previous data centers and building new data centers to fit even more servers into once the ones they have are full.

IIRC, estimates tend to put Google’s current storage capacity somewhere around 10-15 exabytes. Each exabyte is a million terabytes. Each terabyte is a thousand gigabytes. That’s 10-15 billion gigabytes. And they can add storage faster than storage is used up, because they turn massive profits that they can use to pay employees to do nothing but add servers to their data centers.

Google is just a massive force in terms of storage. They probably have more storage than any other organization on the planet. And so, they can share a lot of it for free, because they’re still always turning a profit.

I was recently doing a tour of CERN in Geneva, and they actually still store data on tape because of its cost and reliability over hard drives!

Absolutely huge data centers.

A full third of my towns real estate is currently covered with a sprawling Google data center. Just enormous.

Twitter probably doesn't take to that much space (comparatively) because it's mostly text with some images.

YouTube is another matter. There's an enormous amount of content uploaded to YouTube, as much as 30,000 hours of video uploaded per hour. That's around 1PB per hour assuming most videos are uploaded in 1080p.

I wasn't able to find an official source for what YouTube's total data storage is, but this estimate puts it at 10 EB or 10,000,000,000 GB of video.

On Amazon AWS that would cost $3 Billion per month to store. The actual cost to Google is probably much lower because of economy of scale and because it is run by and optimized for them, but it is still a colossal figure. They offset the cost with ads, data collection, and premium subscription, but I would imagine running YouTube is still a net loss for Google.

Three additional things that you have to keep in mind are that:

1 - Enterprise storage is much, much denser (as in, capacity per physical space occupied) than you would expect.
2 - These systems have capacity recovery features (primarily compression and deduplication) that save a lot more storage than you would expect.
3 - The elements in the infrastructure are periodically refreshed by migrating them to newer infrastructure (think of how you could migrate two old 500GB disks to a single modern 1TB disk to save the physical space of a disk).

As an example about point 1, this is what IBM advertises in their public whitepaper for their Storage Scale systems (https://www.ibm.com/downloads/cas/R0Q1DV1X):

"IBM Storage Scale System is based on proven IBM Storage 2U24 hardware that can be expanded with additional storage enclosures up to 15.4PB of total capacity in a single node and 633YB in a single cluster. You can start with 48TB using half-populated flash nodes or create a fully-populated NVMe flash solution with 24, 2.5” drives in capacities of 3.84TB, 7.68TB, 15.36TB or 30TB. Using the largest capacity 30TB NVMe drives, up to 720TB total flash capacity, in a 2U form factor, along with associated low weight and low power consumption. Adding storage enclosures is easy as up to 8 enclosures (each 4u with 102 drives) can accommodate up to 816 drives of 10TB, 14TB or 18TB or 14.6PB of total raw HDD capacity."

In short, you end up packing a stupid amount of storage in relatively moderate spaces. Combined with the other two points, it helps keep things somewhat under control. Kinda.

YouTube isn't even profitable yet. Google pours billions into storage and compute, so does Amazon and Microsoft and all the others. They have so much space we probably can't even comprehend it

For twitter it's not that complicated because tweets are quite short and text compresses very well. The pictures and videos people upload are of course another story, I'm not sure what Twitter uses as a backend for anything though.

Not only that but for each video on YouTube there are different versions for each resolution. So if you upload a 1080p video, it gets converted to 1080p AVC/VP9, 720p AVC/VP9, 480p... also for the audio.

If you run youtube-dl -F <youtube url> you will see different formats.