How does a bitcoin exchange collect all the withdrawal requests (btc addresses) and send to multiple address as one transaction?
They write a computer program.
The computer program stores up all the requests during a short time and creates a single transaction at the end of that time that combines all the requests into a single transaction.
I know some wallet have manual sendmany functions, but i assume bitcoin exchange will do this automatically?
That's an implementation decision that each exchange will make for themselves, but yes, typically it would make the most sense to have a computer program handle that for you.
Also, i notice some exchanges will have review time. If it's all done automatically, what are they reviewing?
You'd have to ask the exchange what they are doing. Each exchange can decide for themselves how they want to operate.
Most likely the delay after the withdrawal request is implemented so that there is time for the exchange to block the creation of the transaction if they identify a problem.
checking btc addresses or account balances?
That shoule take only a fraction of a second.
