A remodeler will look at the bathroom, the framing below it, and the path of the existing drain. In an older Whitman house, expect a conversation about whether joists can be notched or need reinforcing, and whether the drain can move without a long new run.
Once the tub or old base is out, the plumber sets the valve and drain under a permit from the Whitman Building Department, and the work waits for the plumbing inspector before it's covered. If a new light or fan circuit goes in, the wiring inspector checks that separately.
With inspections passed, the crew builds the sloped base, installs the waterproofing, and should water-test it before setting tile. Glass is usually measured last, after the tile is done, so plan on a short wait for it.