I've wondered about this while using CachyOS and their package installer. I don't know what repos do what, I don't really understand the security model of the AUR, and I wonder, if I download a package, how can I know it's legitimate or otherwise by some trusted user of the community vs. some random person?

To provide some quick information (I implore others to correct me here):

- CachyOS packages should be coming from known, trusted CachyOS and Arch Linux maintainers. There is still potential for them or their original packages to get compromised (See XZ backdoor) however they are pulling source code from trusted sources so you can generally trust these as much as your trust the OS itself.

- AUR packages are a complete wild west. AUR packages are defined by PKGBUILD files and I highly recommend learning how to read PKGBUILDs and always reading them before installation and re-reading them when they are updated. PKGBUILDs for AUR packages can be treated as untrusted shell scripts and to a certain extent an arbitrary actor can make and upload any PKGBUILD to the AUR. Feel free to use them, but make sure A) they are downloading from trusted sources like the original git repo and B) they are running commands that are expected.

EDIT: Improved accuracy.