I saw there's a USB plug on the DJI spark 3 battery power bank.
Is there a firmware update for it? Couldn't find anything in documentation/DJI website.

That is a charging port for your controller with micro cable.

The connector referenced is a microusb connection on the powerful itself and not the power supply.  It is indeed for firmware updates unfortunately.  I tried connecting to a qc2 power addapter and it didn't work