i have this waiting room model in the map and i would like to know how to make it bigger

can someone explain in how to import custom textures. thanks in advance

i dont know how to make the map bigger. like how do i expand the sun/volume/60fpsordie that stuff so

I cant figure out how to make diagonal things like doors and hallways etc. help me plz

I want to make it so you have to buy an ending that brings up a cutscene like how cheesecube unlimited does.

Say i want to work on my zombies map with someone over the internet, how would i go about doing that? Do i send them the radiant file? help me