alt Hacker NewsThe whole point about having an open platform from boot is you don't have to trust it. You run your own code from first power on.
Is it possible that it's backdoored, have a secret opcode / management engine? Probably, but that goes to everyone, as it's not practical to analyze what's in the chip (unless you're decapping them and all)
I don't know what secure environments you're talking about, if it's an airgapped system then you should be secure even when what's inside 'tries to get out'.
Korean and western made stuff guarantee to have such thing. CNC devices in Russia stopped working. Even NVIDIA gpu has back door according to China and NVIDIA had to settle this matter behind the scene with China government. At this point, your phone is 100% backdoorable by western government. The only thing protect you is you are non-threat and too small to be bother with.