I've never used it and just learned that you can store arbitrary files there. They also store videos. Storage is quite expensive and I've heard that Youtube has troubles making money.
So how does Telegram make enough money to support unlimited file storage?
- Premium - a monthly subscription that gives some additional gimmicks
- Ads - they sell context-based ads in some large channels
- Crypto - they developed and invest in TON which used to be fully theirs (maybe still is? idk) and it's incorporated in the app
- Paid features inside the app, like you can buy their in-app currency to then use it in bots and whatnots
I think these are the main ways, also their CEO is a billionaire which is supposed to cover the rest of expenses and can gather investors to help.
Edit: sorry I suck at formatting