> I've never understood the appeal of Debian or Ubuntu, or I guess I don't understand why people would invest so much time in them compared to Fedora and Redhat.
For historical reasons. Debian was around before Fedora, CentOS, and Red Hat Enterprise (the last one I'm not 100%). I also remember growing up thinking Red Hat had a more enterprise focused community, whilst debian was more for the masses. Wrong perception? Perhaps, but a perception nonetheless. Practically everyone I know that used Linux back then did it also for ideological reasons tied to the free software movement. Red Hat was a company, Debian was a movement. It had a social contract: https://www.debian.org/social_contract.1.0 which aligned with most of the free software community's ideals back then. This was a time when not licensing your project under GPL and instead choosing a BSD/MIT style licence was frowned upon because "something something Microsoft BSD TCPIP stack you see?".
I imagine everyone has their reasons, mostly subjective, but the first time I got into GNU/Linux, circa 1999 with Red Hat 6 (pre-fedora days), and Debian was not yet on my radar. Fast-forward a year, and I installed Debian (I think it was "potato") on my machine, and fell in love almost instantly thanks to apt, their ethos, and just the way the entire distro was designed (relatively easy switching from stable, to testing, to edge...). Mind you, I was a very novice user and not that tech savvy, but I was (I still am) a "believer" in the free software movement. I only had a 256kbps internet connection and used to download packages sporadically. The Debian CDs came with quite a good number of packages, and I remember magazines that distributed CDs with entire repositories (you could add your CD as a repository source).
Fast forward a few years, and I had left Debian for Slackware, then FreeBSD, then Gentoo (they lured me with the "portage is like ports" hype), and then Arch Linux. I always kept my respect and admiration for the good experience I had with Debian, as a user. I never felt that for Red Hat. What always took me from one distro to the next was tooling. I'd say tooling is the number one aspect of user experience in linux distributions for a certain type of user which tends to be less tech savvy than the rest. People want to install packages that auto-configure, and then tweak if necessary. Debian gave you that (sort of). You even had unattended upgrades. Back in the early 2000s that was a real "OMG" feeling for some, myself included.
> I've also never understood the appeal of .deb or apt.
The same reason Homebrew is such a success in macOS, or chocolatey for Windows.