I know GA using UserAgent string to detect device id or brand name for an android device. But i wonder do Google using WURLF library or they building and device's useragent database for themselves.
Because our company has an android device that update user agent to WURLF but in GA, it's still not set.
Have you looked at your user agent data in OpenDDR?
These slides talk about where Google is looking to go with acquiring mobile meta data and a proposal for using Client-Hints
However, as far as I can tell in web analytics, devices/browsers that don't have identifying UA strings are simply not going to be counted by web analytics companies, and hence not catered for.
Some somewhat relevant discussion here.