The base OS of Android is, but the widely dispersed version maintained by Google with the hardware drivers required for it to work with the actual hardware is not.

That's why projects like Graphene can't work on every phone. They have to rely on either reverse engineering hardware drivers or the manufacturers providing the drivers. For many years, Google openly released the drivers for all the hardware in their Pixel line of phones, which allowed Graphene and other customized versions of Android to easily work on Pixel phones.

Manufacturers usually don't release drivers separately and instead they're only available built into the manufacturers customized Android version. Android Open Source Project is the open source base, then Google builds their proprietary stuff on top as "Android", then the various phone manufacturers build their own versions on top of Google's "Android" with: manufacturer specific crud added, phone specific crud added, and often phone carrier specific crud added.