The remodeler removes the tub, opens the walls to the studs around the alcove, and checks the subfloor and framing for water damage. The drain usually shifts to suit the new shower base, and the old tub valve is typically replaced with a new shower valve rather than reused.
In Bridgewater, the plumber files online for the drain and valve work, and the plumbing and gas inspector looks at the rough plumbing before the walls are closed. If the job adds lighting or a fan circuit, an electrician pulls that permit. None of it should start until the permit card is in hand.
Much of the calendar is waiting for inspections and materials rather than labor. Ask for a written schedule that shows demolition, rough inspection, waterproofing, tile, and glass, and ask how the room will be left between stages.