Dumb Canadian here trying to figure out how this pool works.

Howdy pool people. My in-laws recently purchased a house in the palm springs area and have a pool on their property. I live in Canada, so have very zero knowledge of how pools work. I think I figured out how to heat up the pool, but somehow the hot tub level went way down. There seems to be warm water flowing into the pool though. I know this is a big ask, but can anyone give me a few tips on how this system might work?

Thanks so much!