Tile is not waterproof. Grout is not waterproof. A shower stays dry because of the membrane and the slope underneath, and that work is finished before the first box of tile is opened.
Pitch, then membrane
The pan is sloped a quarter inch per foot toward the drain, then sealed with a bonded membrane that runs up the walls above the shower head. Corners and the curb get pre-formed pieces so there are no seams where water likes to sit. It is the same system behind every custom shower and tub surround we build.
Flood test
Every pan is plugged and filled for 24 hours before tile goes down. If the water line moves, we find out now, not after the walls are finished.