The Purism 5 is going to be using a baseband processor that's similar to ones used in car infotainment systems. It won't share RAM with the CPU and will be isolated from the rest of the phone. It'll even have a hardware kill switch, where you can physically sever the circuit, if you're that paranoid!

The CEO of purism did an interview where he went to a moderate degree of depth on the technical side, you can watch it here if you're curious. He mentions all of the above features in it, and more!

https://www.youtube.com/watch?v=4SwE9W8JasA

The baseband processor isn't started as first boot, and as I understand it can be selectively enabled/disabled while main cpu is on.