I recommend watching Chris Dumas' videos from defcon, he demonstrates that hardware backdoors do exist and that it is possible to find them, although it is a lot of work.
On the other hand I think the groups that have access to the hardware backdoors are very small and distinct from that of the typical OS backdoor.
If your device is compromised encryption is irrelevant they have access to the keys or could read the data after it is decrypted.
Use a custom OS like Grapheme OS.
If your device is compromised, ROM is irrelevant.
The hardware isn't compromised, the software is.
Android and Apples iOS are shitstain spyware.
I recommend watching Chris Dumas' videos from defcon, he demonstrates that hardware backdoors do exist and that it is possible to find them, although it is a lot of work.
On the other hand I think the groups that have access to the hardware backdoors are very small and distinct from that of the typical OS backdoor.
All modern hardware is completely compromised at the chip level - the OS/Software isn't even aware and are irrelevant.