Most pools would have a service that comes in and maintains the pool. They are also probably doing the maintenance when you are not there. Like early morning prior to opening or later after closing. Way back in the day (over 40 years ago), my sister was a lifeguard at a pool and did perform testing, vacuuming, brushing, and general maintenance of the pool. When I had my first pool (gunite), I was religious about brushing once a week. Once I got a robot, I did not brush nearly as much as I should have. I now have a vinyl pool with a robot and I do not brush hardly at all, but probably should.