Earlier quoted context omitted.
How about the elephant in the room? Radio firmwares. It is meaningless if you install firewalls, if you compile android on your own, only to have to turn around and use proprietary, closed source libs to access proprotary, closed source radio firmwares. Radio firmwares which often have full RAM access, parallel CPU control, GPS and full hardware access, and make intel's management engine, and closed source bioses loo…
Do you really think the adtech industry is using radio firmware backdoors to track users? It's theoretically possible I suppose, but several steps too far into conspiracy theory territory for me to worry about. Governments spying on dissidents is another story.
And with full control of the OS, of where kernel modules memory space is assigned, you'd only have to scan certain buffer regions of memory, to snarf contents of buffers, etc.
The problem is, I don't know, and can't. It's not open source. It is not under my control. I personally don't care about paying, but I do about seeing.