At least with the vendors I’m referring to (2/3 that make all Android phones), they just took the open source code, hacked it up as quickly as possible to get some basic drivers working, and moved on.

There wasn’t any “special sauce” in the source, they just didn’t want to spend the effort to upstream it

Edit: Just because you said “hardware open source” I wasn’t advocating for open hardware, just for hardware vendors to, ya know, support the hardware