So, I can’t install aur packages via pacman?

Nope, you have to do it manually or using an helper that abstracts the manual work away.

AUR packages, or to be more precise the PKGBUILD files, are recipes to compile or download stuff outside from the official repositories, manage their deps and installing them on the system.

You should always only run PKGBUILD files that you trust, they can do basically anything on your system. Checking the comments of the package in the aur repo is a good practice too.

Also Are you quoting certain nExT gEn gAmE guy?

…maybe