* They're an exchange, matching orders.
* They're a retail broker, taking orders from customers and holding funds.
* They're a custody institution, holding both fiat and cryptocurrencies.
* They're banks, lending to others and receiving interest, and borrowing from customers and receiving interest.
* They're traders themselves, for their own account.
Now, in the real world, all of those functions used to be done by separate companies. With "deregulation", there are banks which have brokerage, custody, and trading units, and they do get into trouble. Which is why those are all highly regulated industries with a lot of inspection, required disclosure, and insurance backup.
You'd think the "decentralized finance" people would have figured out a way to separate those functions by now. But no.