choose PureOS if 100% free-software is your goal and typically if your hardware can function properly without binary blobs in the firmware or proprietary software.
also look at how the lower levels of the code can be a problem for you …
choose Debian if you just want a universal operating system which is VERY stable (Buster) and can be enabled to install copyright restricted software from the non-free repository.
one makes the decision for you and tries to make it hard for you to use binary-blobs, proprietary-software (PureOS ideology)
the other is much more lax in terms of how it tries to guide you into something you may or may not want (Debian)
then there are the DIY (do-it-yourself) kind of GNU/Linux meta distributions that are THE BEST but are harder to learn how to setup (Linux-From-Scratch, Gentoo, Arch, Slackware etc.) and are considered THE WAY to rock GNU/Linux like a boss
Linux is just the kernel. there are other kernels available but none are as “mainstream” as Linux is.
see the gnu.org sections and the FSF articles describing the differences between Linux and Linux-libre kernel …
RMS is the father of free-software and a genius so his way of dealing with computer-software problems is probably much more elevated than yours or mine considering that he wrote gnu/emacs from scratch and is a LISP wizard …