We cover mobile security and mobile network security on The complete cyber security course. Volume 3.
If you are looking for security the iPhone is the best choice. We know intelligence agencies have struggled with getting into them with there closed system and security enclave. https://www.theiphonewiki.com/wiki/Secure_Enclave
Custom roms are ok but you have to trust the developers or audit it yourself or see a third party audit of the code.
Android q has improved security. https://www.stationx.net/android-q-new-security-features-unveiled/
Privacy is hard to achieve on phones as there nature is to keep you connected.
I like the Librem 5 for a security and privacy focused phone.