At the lowest point
This is the hydrostatic valve. If the pool is emptied it is removed to relieve water pressure caused by the surrounding water table If not removed a pool can float out of the ground.
There is no filter at the bottom of a pool. Perhaps you are looking at the hydrostatic valve which is there in case your pool is emptied and Will make sure that the pool wont float in ground water.
Any pool can have a hydrostatic valve and nothing has been made that cannot be broken eventually.
At the lowest point on the hot water distributing pipework system
The Hydrostatic valve is usually on the lowest part of the pools floor. The hydrostatic valve is there to protect the pool from being lifted by ground water in the event that it is empty. it allows the ground water under the pool to get into the pool thereby stopping it from floating out of the ground on top of the ground water.
Aortic semilunar valve
If you have such a valve, it was installed as an aftermarket modification. Check with whoever installed it.
A Hydrostatic valve is basically a plug that stop water coming down out of the pool, but when the water pressure under the pool gets higher then the pressure in the pool it will let water into the pool from underneath in order to stop the pool from floating in ground water.
exspanion tank or relive valve
you will have to dig it out and put it back in the ground check for broken pipes and is at all possible install a hydrostatic valve so it wont happen again