Welcome
there are many ways to do this, as many ways as there are different people, this is what makes Skoolies so great
It starts with how you will use the bus, we hate cold so insulation in the walls was not a concern for us, others live year round and want to be comfy
Lots of folks just attach 2x4's down the side metal walls horizontally (kind of like chair rail), using self drilling/tapping screws (first couple pages of my build La Tortuga, should give you an idea)
This lets you attach everything to the walls with screws
Others will post I am sure, also look at Bertha thread they used metal stud hangers, others have also
Again, welcome