Here we just talk about btc. First, the trading platform will pre-generate a (multiple) BTC address for each user.
Then you send a transaction to that address, and the exchange detects the change in the balance of that address until a certain number of confirmations are made.
Verify that the transaction has arrived and write the balance to the database of the account.
For security, the account address may be aMultiple Signature AddressMultiple signature addresses are at least a few private keys to use the address’s btc.
Exchange currencies are usually huge, they will put money in their cold wallet regularly, and their BTC address is actually not btc.
If cash is withdrawn, it is withdrawn through the hot wallet of the exchange.

Don’t really think of a Bitcoin wallet as a wallet.
Bitcoin’s entry and exit are related to utxo. In fact, utxo is really used to make transactions. As long as you know how many utxos are transferred to your account in the latest transaction about your account, you will know how many bitcoins are in your account.
In fact, you just need to understand the concept of utxo.

In general, Bitcoin is a memory field, but it is the only one that can’t be copied and has no backup.
When you pay the money, people give you field files, and then you save them and trade them.
Many people have lost this document, lost it or lost it, and never again.
Above