Probably under the white brick since it's no longer used as a pool. The lit edison bulbs in the deep end indicated as much.
Yep.
It was a pool at one time though. The last time it was used was probably in the 70s or 80s. Looks like subway tile was laid over the return lines and main drain using subway tile.
If anything that would be the fill line. Pool drains most likely were covered with subway tile when the pool was retired and used as something else.
Main drain was most likely tiled over using the white subway tile. Pool is at least 40,000 gallons. No way a main drain for that pool is a little 1.5 inch x 5 inch slit.