EDIT: I kinda solved it by installing Wayland (with Nvidia card, Ouch!) to replace Xorg. Not sure if this is gonna last though. Perhaps Manjaro is the one I'm gonna throw out FIRST if anything happens from now on.
What should be the first line of defense? Timeshift?
This happened after I installed AUR package masterpdfeditor and 2 applications from github (some hashing algorithm programs, I think they were "Dilithium" and "Latice-based-cryptography-main", one of them was provided by NIST.)
If using GUI: I login, black screen for few seconds, then back at login screen.
If going to ctrl+alt+f2, login successful, then startx, see picture provided (higher quality).
I tried adding a new user, but result is the same.
I have a live usb to do the Timeshift. (I can also chroot if necessary... But I'm not extremely professional)
It may work but there's no point in doing it. You get something that's neither proper Arch nor Manjaro.
Manjaro is built around a branch which doesn't exist in Arch, unlike other Arch derivates, and mixing the installed Manjaro packages with Arch packages can lead to unpredictable results.
That's why you change the repos, tho, instead of mixing those from arch and manjaro, and do overwrites to avoid trouble with their configs. Also, I have a feeling pacman tells you when a package managed by it is no longer in the repos, so you just remove it to not accidentally take part in another round of ddos-ing aur or whatever manjaro's packages currently do for fun.
As for why, that's just to avoid setting up everything from scratch