so the available software is a bit limited then

I can still customize and configure things, or not really?

does that mean I can't customize the OS, like shortcuts?

The way SteamOS works is a bit weird. By default the OS partition is read only, you can do whatever in your user directory. But they let you unlock it to make whatever changes you want.

I’m pretty sure that this OS partition gets completely wiped every update though so your customisations will have to be reapplied.