The entire floor is concrete but the shower area was ground down to expose more aggregate.

i wonder how was the concrete waterproofed? was a liner used?

Was a shower pan and liner used in this shower?

Unfortunately, I am re-building 3 showers just like this in my brand new home. Originally no liner was put in and water leaked under the walls. My concrete is getting sawed and jackhammered out. They are digging down 6 inches. The first layer will be 2 inches of concrete then the pre-slope pan, membrane and then 3 more inches of concrete. Use a very hard concrete or put mesh in it to prevent cracking. The liner will also help with preventing cracking (consulted architect). Use an FHA drain with weep holes just like in a normal shower. You will need to adjust the drain up because the 3 inches of concrete is much thicker than mud and tile over the membrane. If I were building from scratch again, I would frame out the showers when pouring the slab. I would also pour a six-inch stem wall around each shower so the concrete is one piece between the floor and wall. The frame on would set on the stem wall. If you don't want the aggregate you can use muriatic acid to etch the floor then seal. There is a misconception that concrete doesn't leak. It does leak (capillaries) and it cracks. With no membrane, the water has nowhere to go and you will eventually get mold.

Sorry to hear that. This is definitely a demanding detail.