How exchanges manage funds and addresses
There is no "user's wallet" when you are using an exchange. What you might think is a "user's wallet" is actually the exchanges wallet. When you deposit any cryptocurrency into an exchange, you are not depositing to "your exchange address", you are depositing to an address owned by the exchange (thus part of the exchange's wallet) and the exchange promises to give you the same amount of cryptocurrency that you deposited to that address if you want to withdraw from the exchange.

When you buy or sell on the exchange, no actual transactions happen; they just update some entries in a database. No money is transferred anywhere; it is all held by the exchange in their wallet.