Checking where package comes from should be easy, replace last word with repo name but keep '^':
pacman -Sl | grep "^system"
pacman -Sl | grep "^world"
pacman -Sl | grep "^galaxy"
pacman -Sl | grep "^extra"
pacman -Sl | grep "^community"
Yeah packages creeping in because of -Syu with newer versions is significant, even non-related packages get overwritten if they are newer on Arch. I think I'll stick with PKGBUILDS if package that I need doesn't exist on Artix.
Edit: hang on while I check commands above, it seems I am querying where is what in repos, not what I installed. One sec.
Edit2: Yes, above commands are querying what's available, for a moment I thought that's what's pulled it. Give me few days and I'll write proper analyzer where each package comes from and open separate topic.