bitcoind – how does the deposit / withdrawal system work

I am trying to understand how the exchange works and other platforms that use bitcoins with users. Especially the deposit / withdrawal method.

For now, what I know is that they have to use api as bitcoind.

Remain on bitcoind (that's the maximum I've read for now), they create an account in the wallet for different users (that's ok) but after users drop / remove bitcoin, they use after that I've read the "wallet inform" order to find out when a new transaction is in progress, but how do they handle the database information?

Do they save information on the database or do they simply handle the bitcoind command?