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Exchanges are 2 types : centralised and decentralised. Now, if any new user is coming to the centralised platform then they need to deposit BTC (taking BTC as an example). For depositing, each time exchanges create a public key(give this to user to deposit) and private key(hold by exchange). Now, let user have 10 BTC in his wallet (Which he can check in bitcoin explorer). In a day, he traded his 2 BTC against ETH. This 2 BTC he traded with 10 people. So, now his actual balance at his public key would be 8 BTC. And all transaction which he did with 10 people will also get reflected at bitcoin explorer. Trading also works instantaneously. it means as soon as someone sell or bought BTC, his balances get chcngaed accordingly BUT bitcoin actual transaction take 10 minutes for getting first confirmation. So, how the actual system is working behind the scene?