When you bitcoin on an exchange, you are actually buy bitcoin from other users, not the exchange itself.
You deposit fiat and place a buy order. Someone else deposits bitcoin and sell it to you. After that, you will be able to withdraw bitcoin and the other user will be able to withdraw fiat.
So, for every satoshi added to a user's balance, one satoshi is deducted from another user's balance.
tldr for OP just for more clarity; it's pretty much like eBay — but instead of selling random stuff, everyone's buying and selling crypto. Exchanges really don't sell anything, they just charge an exchange fee for people buying and selling coins to each other. Just like eBay!