Modems are usually directly connected to the battery rail, because (so the saying goes) they have high peak power consumption.

I once built a cell phone from scratch. They do have high peak power consumption, and I did connect it to the battery rail directly, along with some capacitors to handle the surges.

I have a Pinephone, which has hardware switches to turn off potential privacy destroying features.

But it's even more secure than that - them made it so it's such a poor phone that I never managed to get it into use as a daily driver phone for more than a day or two at a time! The most private device is one you never bother carrying!