There’s a recess in the tiled shower wall for your soap.
The floor of this recess is a special tile (or tiles) that is a grate.
The grate leads into an appropriately lined shaft within the shower wall, behind the tiles.
This shaft empties out into the shower area via another special tile.
I hate it when soap sits in its own soap water. There are soap dishes which elevate the soap above its water, but you still have to empty them. This eliminates the issue. It could be adapted for sinks as well.
There are also similar raised soap dishes which have drain holes in the bottom. Or whose catch chaber is large enough that the drip water will evaporate before the next use.
I have one of each and they’re nothing exotic; just Target’s finest.
No need for some complicated plumbing inside the wall.
There are dozens if not hundreds of soap holder with drains. I have never heard of one that used a tube inside the wall and cannot imagine any benefit to it.
OK, if this is a problem, pro tip: stick a bottle cap into the bar of soap. Then it will not touch the surface it is sitting on or the water, assuming the soap holder or shelf or bathtub rim is otherwise draining.