github is a service for git from m$, gitlab is both of its own service and a product that can be copied for doing basically the same game on any server, and both purism and postmarket use gitlab, but git is the main thing, it is the version control system (vcs) as their core part… when u want to download from these sites, u have options for downloading a tar/zip archive, or using git at your terminal, where u can use https or git as protocol, where the former is accessible without an accout on any of these 3 mentioned and other git-based platforms, while the git protocol requires you to set up an account with an ssh key for authentication, and the latter protocol provides more abilities… i guess this is what u saw there… but im nowhere near to be a git expert, just had some half-baked attempts to play this game from terminal…
i use postmarketos with sxmo on my pinephone, i know they support librem 5 too, but not how much readily and as expected… im yet to familiarize myself seriously with pmbootstrap, but it should run basically on any sort of linux… i would work out my way myself the hard way, but i suggest u to find an instant communication channel to them like irc, and ask for help/resources/troubleshooting there, as thats probably the fastest way to achieve what u want…
sxmo is nice, hacker friendly, and it saves much battery, but it has a learning curve, and we cant expect purism to go that way, and make a phone mainly for hackers, but making a real linux phone is like that currently, no matter how much sugar is added… u need patience, and to go near to the right community (i like this community, but i mean where the devs hang out
… or discuss their development stuffs…) everything else can only boost and make u more independent, but this is fine… nobody seriously need to know english or coding, but patience, trial&error, cooperation and orientation…
btw dd isn’t required for flashing, when there is curl (or wget), and compressors can operate on data streams, u can flash iso’s with curl, right from the internet to its destination, u still only need to take serious care about its destination… like i did this:
curl https://images.postmarketos.org/bpo/v23.06/pine64-pinephone/sxmo-de-sway/20240122-0322/20240122-0322-postmarketOS-v23.06-sxmo-de-sway-1.14.0-pine64-pinephone-installer.img.xz | pixz -d >/dev/mmcblk2
just don’t take it literally, but as an example…
and the tools shoud be installed… 
note that i had broken stuffs here and there from time to time, i guess debian (behind pureos) is way more stable, but thats just a different trap…
devuan (no-systemd debian) probably has a larger team than purism, and they always lagged behind debian seriously, and when i used debian on a small partition, then point release filled it in the middle and then nothing good followed… btw i use void linux as my main os, but i like alpine too, just not that much, but maybe 1 day i will enjoy it on my pinephone too…
(while i will see alpine on my visionfive 1.0 (my other gem) sooner than void in place of the current ubuntu…..)
back to installing pmos, u should know that there is/was a rewrite of the installer, and having full disk encryption again means u used the newer one, i had to download an older iso with the old installer when i realized this (absence of fde), and then i could update it, which was new at the time, otherwise ppl reflashed it at every release before that X’D
back to pureos, and the choice of depending on debian, i think a rolling release distro with removed proprietary components could be better for the case of too big point release upgrades, but their development branch could be used the same way with package pinning/downgrading, and/or with any means of system rollback (btrfs/whatever
) can do the trick, just go nearer to the development in case of need… otherwise they did most of their work on their apps (or i believe so), which are packaged elsewhere too, and even if they have the debian base for main development, they could open up their path to other distros, so they could win independence, but sure it is continuous extra work, and they are a small team, but they have a big community, and many ppl around already took much of these steps, and there are means to contribute howto/code… it can only become yet another selling point…
btw, purism folks, for ur benefit, u have this awesome community, guide them toward contribution as ur second nature, and u will see how much manpower u really have!
like by answering with links to opened issues with cross reference to the forum, and then they will see how things work “behind the scenes“… already having things/methods like this is one thing, including them in ur advices/habits is an another… on the contrary, maybe it is better to keep most of the community on the forum, as it would increase the noise level around development, which might slow it down, but i still guess that it can be done right…
+maybe some other things i forgot to write, but wanted to… 
(edit: minor corrections)