I guess this could also be based on the distro you use as well as your graphics card. For me, I use EndeavourOS, which is very close to base arch, so I had to do some extra setup to get proton working on it. For some reason, Proton refused to work on the Arch repo’s Steam package, so I had to use the flatpak version instead
The only thing really preinstalled is basic stuff like desktop environments and a few tools to help with updates and manage the system (eos-update, etc). Even almost all the package repositories are the ones maintained by arch.