Waterproofing is the part nobody ever sees once the tiles go on, which is exactly why it gets rushed on so many jobs. It is also the only part that actually stops water.

Applied after falls are set, not over a sloping mess
Bond breakers and reinforcing at corners and junctions
Every pipe penetration sealed individually
Left to cure fully before any tiling starts
Bathrooms, laundries, balconies and podiums

The substrate has to be sound, dry and falling towards the waste before anything is applied. Membrane over a floor that ponds will simply hold the water in a different place.

Floor to wall junctions, internal corners and pipe penetrations get bond breakers and reinforcing fabric, because those are the points that move and those are the points that fail. The flat areas almost never fail on their own.

Then it cures. Tiling over a membrane that has not fully cured is the single most common reason a brand new bathroom starts leaking, and it is entirely avoidable by waiting.