How does power bank know what voltage and amperage to use ?

Some power banks have an adjustable output voltage. How does a power bank know what voltage and amperage to use ?

Advanced power banks can exchange information with some connected devices about their charging capabilities.

So power bank can "ask" connected device what voltage can be used for charging. If connected device is able to provide such information, then a power bank will use a voltage that both of them support.

The feature that allows powerbank to ask what voltage and amperage can be delivered, is called Power Delivery. It is known also as PD.

PD is a so called Smart Handshake between powerbank and charging device. Base on PD, powerbank can figure out the exact amount of power the charging device can safely handle.