if it has a locked bootloader, you're probably SOL.
AT&T and Verizon lock their bootloaders on their phones. Yep, it's their phone not yours, you just pay to use it.
You actually don't need to root it at all to install a custom ROM, you will need to unlock the bootloader (and use a phone that even lets you do that in the first place).
It's largely a moot point.
If you were putting tracking in, you'd put it in the baseband processor.
it depends on the phone.
if it has a locked bootloader, you're probably SOL.
AT&T and Verizon lock their bootloaders on their phones. Yep, it's their phone not yours, you just pay to use it.
You actually don't need to root it at all to install a custom ROM, you will need to unlock the bootloader (and use a phone that even lets you do that in the first place).
Here's a good Youtube channel for this kinda thing: https://www.youtube.com/watch?v=xIXAzA555xk