GrapheneOS

GrapheneOS is a fork of Android. It's more secure and private.

Graphene seems to be the most stable 'alternative' mobile OS out there right now (April 2024).

Installing

You need an Android phone. If it has files on it, back them up first.

Then follow official instructions. I suggest the WebUSB method, if your browser can handle it.

Disadvantages

Some banking apps don't work on it. They think that your phone is insecure. Check if yours is supported before moving to the OS.

I also couldn't get Pikmin Bloom to work (September 2023).