The preformed liner functions similar to a swirl filter. I am not a fan of the swirl filters personally. The GB will function similar to a filter, but they are by no means the only GB. These are just the ones I am working on adding.

The water going from the pond to the 330 IBC filled with Bread Racks for the shrimp doubles as a settling tank. That's why the water comes in from the bottom.

What you are talking about with the old style garden is what the hippies did with catfish ponds back in the 60-70's and I believe one of the major contributions to guiding AP to what it is today. IF you do that, which is fine, it works fine. But it's no longer a "closed" system, but a semi closed loop.

Rain water is smart, it's how I top off my system as well. I buried a 330 gallon IBC in the ground to hide it, then ran gutters to it with wire mesh to prevent crap from getting in tank. Then use harbor freight shallow well pump with foot valve to pressurize garden hose. This allows rainwater to go through sprinklers or whatever I hook it up to.