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?
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.