Removal comes first, and cast iron tubs are often broken up in place to get them out of a narrow old bathroom. Then the remodeler and plumber look at what's underneath: the drain line, the trap, the subfloor, and the joists. Expect a conversation about anything that should be replaced while it's exposed.
The plumber pulls a plumbing permit from Marion's Building Department, and the valve and drain are inspected before the new walls go up. Inspection days are limited, so the order of work matters; ask how the remodeler sequences demolition, rough-in, inspection, and waterproofing.
After waterproofing and a water test, the walls are tiled or paneled, the trim and glass are set, and the shower is ready once grout and sealants cure.