This guide from Grugq is a great overview for phone privacy and applies even if you have a phone with hardware kill switches for the modem, or a user-swappable modem like in the Librem 5. While those things might make certain steps easier, you still have to keep all the correlation in mind.
https://grugq.github.io/blog/2014/02/10/a-fistful-of-surveillance/
Since that article was written there's been additional research on using other sensors (ambient light, gyroscope, etc) for tracking, which is why we implemented lockdown mode in the Librem 5 so you have an option to turn it all off.
