They either generate wallet address dynamically, and in those cases allow you to regenerate one at will.

Or, they have wallets that acts as combined input/output for all/groups of users.

I don't know a single big exchange that generates exactly one wallet per user and uses that one always for the same user.

When you think about it, it makes a lot of sense they do it like that.

First one maybe but definitely not the second one. Have you actually thought of it? How would the platform know who sent what. They can't. So it's definitely not shared on the deposit side. And I doubt they'd move the money around because of fees so I doubt it's shared on the withdrawal side too. From my experience, of course it depends on the blockchain used, but they all have static addresses per user

Addresses, sure, but not wallets.