Postmarket os is required if your device runs android 7 or below due to termux limitations which prevent you from installing packages, or if your android version has missing kernel syscalls that prevent you from running containers without x86 emulation which is slow.

Rooting is required to install postmarketos, and making termux faster and bind to ports on Android. You don't need postmarketos if your android version can run containers on termux with all the required kernel syscalls

To root or install postmarketos you need an unlocked bootloader.