distro repos: there is a process that is supposed to ensure no malicious changes make it through. Usually far enough behind recent code changes for new issues/code being compromised to be spotted
official package outside distro repos if packaging org has secure release workflow
building from source / official package on external repo if you know little about packaging org: malicious contributor or a compromised account is enough
unofficial package: like building from source, but you have to worry about package maintainer too
Don't blindly run untrusted software, use Bubblewrap at the very least. Keep https://xkcd.com/538/ in mind.
Do you categorize AUR packages (if you didn't verify the PKGBUILD on every update) as untrusted?
Yes. AUR package maintainer(s) are additional people who can add malicious code (or someone else can by compromising their account).
I know that almost nobody treats it this way but the number one rule of AUR is that it's pretty much all untrusted, by definition.
Same goes for any unofficial flatpak, right? And that is most of them.
In order from the most to the least secure: