If the CIA wanted to put a keylogger on your phone, it would be customs, the CIA would intercept it at some point, possibly in customs, then put it in the boot loader or on the SSD firmware or something so you’d never know it was there and wouldn’t reasonably be able to overwrite it no matter what.
That probably doesn’t help you though.
If you want to be sure you’d have to reflash every bit of the phone, not just the OS.
If the CIA wanted to put a keylogger on your phone, it would be customs, the CIA would intercept it at some point, possibly in customs, then put it in the boot loader or on the SSD firmware or something so you’d never know it was there and wouldn’t reasonably be able to overwrite it no matter what.
That probably doesn’t help you though. If you want to be sure you’d have to reflash every bit of the phone, not just the OS.