Nothing is free and I haven't quite figured out how these large APR yields are being generated and how long they are sustainable.

Generally? They take the deposits and make very risky loans with them.

I mean, that's how these things have generally been set up. You give them your money/stablecoins, and they lend that money/stablecoin out to folks. Often in the form of providing leverage to their other customers making bets on margin on other crypto. There might be a DeFi platform that's doing something different. But that's usually what's going on.

You get a super-high yield because they're making risky loans with your money, and you might not always get it back. These aren't banks, they're not insured, they (often) have relatively aggressive approaches to risk management. You get great returns if everything goes well, but you might not be able to get your money out if things don't go well.

For more detail, I cannot recommend enough Matt Levine's now-older tome of an article (but still good) on all things Crypto and Defi:

https://archive.ph/jFaGB