An exchange does not sell you crypto from its own pocket. It matches your order against another user's and keeps the record of who owns what inside its own system.
This explains why balances move instantly on a platform and slowly on a blockchain: internally it is a database update, and only a withdrawal touches the network. It also explains why deposits and withdrawals are the moments that need care. Inside, you are relying on the platform. On-chain, you are relying on the network. Those are different guarantees, and every transfer crosses between them.
Binance Academy covers this in more depth: https://www.binance.com/en/academy/glossary/exchange
Trading crypto from Dubai since 2019.
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