I'm in the process of building one myself. Take what I'm saying with a massive grain of salt, but I believe you basically need three things - objective, tube lens and a camera module.

If I were you, I would probably buy a normal microscope that supports a microscope camera attachment, instead of specifically looking for USB microscope. There is a lot of misleading marketing around USB microscopes, a lot of them are literally just doing digital zooming in (I have a 30€ "1000x" microscope from Amazon which is nice for soldering and such things, but these fake microscopes appear even at higher price ranges). Treat anything >1000x for light-based microscopes as a red flag.

Some random ones I've encountered (less for PCB work though):

- https://openflexure.org/

- https://www.eurekamicroscope.com/

Perhaps better to ask here: https://www.reddit.com/r/microscopy/. They get that question a lot.