Chromebooks have no room for manufacturer involvement with the software. Afaik, assuming based on public references, if you want to make a Chromebook, you use Google's reference design for the motherboard, pick a compatible CPU, design your own keyboard (but use a Google approved controller instead of a normal one), design your own case and pick a display, put everything together, use Google's firmware and use Google's ChromeOS build. OS updates come automatically from Google.

Perhaps it's slightly less rigid than it seems, but I've seen several Chromebooks and they all seem exactly the same. There's not even a manufacturer logo on boot.