Installing Steam

Steam is a native 32-bit application, while Artix is a 64-bit Linux Distribution.

Artix's lib32 and Arch's multilib repositories, both contain 32-bit software and 32-bit libraries that can be used to run and build 32-bit applications on 64-bit installed systems (e.g. wine, steam, etc).

Therefore you need to enable both the Artix [lib32] and Arch [multilib] repositories in /etc/pacman.conf

See the corresponding Arch wiki page for further information.