I don't care if anyone has a Xiaomi, Oneplus, Samsung, etc. Each brand is using a modified version of Android, and they chose to be compatible with each other. But for example the "blue vs green bubble" drama is a thing specifically because of Apple locking their unsuspecting users into a closed ecosystem. And it sure isn't Android's fault for not being compatible with it.
The more power a company like this gains, the worse will it be for the whole industry.
Isn't android built off Linux?
It technically is, but "Linux" in Android's case is meant as the kernel.
In a rather unorthodox way, yes.
Android is one of those rare examples of a Linux kernel not being paired with GNU tools. I believe Android wrote their own versions of all the tools they wanted.
The kernel is also extremely locked down by default. They very intentionally designed the OS in such a way that every facet of the kernel is kept abstracted away from you. It's about as black-boxed as you can get, to the point where the fact that it's Linux underneath is almost meaningless.
And MacOS is BSD :)