Shower pans come in three main types.
All shower floors just naturally leak.
Water makes it through the grout and even through some types of tile itself.
Then what? The shower pan takes over.
It will be one of these...
Masonry pans.
Built up in layers, old style pans work fine as long as every step is performed properly.
Problem is there are many ways to go about it and even pros often disagree.
The basics include two layers of mortar.
The lower layer is sloped.
Then over it goes a liner membrane sheet.
That's the place any seeping water stops.
Over the liner goes the top mud that's the base for the tile itself.
What about an easier way...
Kerdi.
Kerdi showers come standard sizes and custom too.
The pieces are a bit different, but the principle's the same.
A sheet membrane fit all around the shower becomes the place the water stops.
Some models use no mud.
In others mortar serves just as a base.
In either case, showers go together fast with little or no mud skill required.
Pricey though, very pricey.
Tile Ready or Tile Redi.
Cheap fiberglass or plastic shower floors won't work as base for tile.
Too much moving of cheap floors means cracks and failed tile.
That's why tile ready shower pans are heavy, stiff and able to hold up to loads without moving.
Maybe backed with mud, but only for support, that's all.
Standard sizes mostly, though customs can be had for a price.
Shower pans are key to leak-proof performance.
All these types will do.
If price matters, you may choose mud work.
But trouble-free often calls for one of the newer types.