Dumb questions (sorry in advance)... Who builds the tub surround? Is it the tile guy? The plumber? The GC and his framing crew?

Then after it is framed, the tile guy puts cement board down, and then waterproof the cement board - what kind of waterproofing and who does this? Then tile is put in. So I attached a picture of something I think we'd like to do. Where the tub surround runs as the bench in the shower. I read on another post in this forum that the shower pan should run under the bench. But I really don't understand how that would work?

Or is it a lot better to have a tiled floor instead of the pan?

Modern Bathroom by Athens Architect Bork Architectural Design, Inc.