A UAP (as well as the UniFi controller) in its factory-default state can have two possible default username / password combinations:
In version 2, after adoption, the username and password (for SSH, etc.) will correspond to the administrator username / password as configured in the UniFi controller.
In version 3, because of multi-sites, the device login username and password can be found under "Settings" panel > "Site" > "Device Password". From there, you can also change auto-generated password to something easier to memorize.