Per the Polaris installation manual, the inlet to the booster pump is tapped from one of the return lines going to your pool. The illustration shows this tap into the return line is above ground and before the chlorinator. This is why there is always flow through the cleaner line when your main pump is running because it is getting pressure flow from the main pump through the booster pump and out the line to the cleaner. You should see some flow out of the back nozzle of the cleaner when the main pump is on and the booster pump is off. When the booster pump kicks on, it then increase the pressure flow to the cleaner to push it around the pool.
My connection tap on return line is underground as all I see is a 1 1/2" pipe protruding from the ground which is then crossed over to 3/4" to tie into the suction side of the booster pump. I did not construct the pool so I have no knowledge of its original layout. The return side of the booster pump has a dedicated line to the pool (sidewall) where the Polaris hose attaches.
Hope this helps understand the flow paths and how it may apply to your set up. Clearly every PB may have their unique way to plumb in a pressure side cleaner; however, the suction side should connect to a return line somewhere either on your equipment pad or underground. My personal preference would be to remove the booster pump, connect the suction line to the return line and just use that line as an extra pool return. It is already functioning like that now except it is flowing through the booster pump and through the cleaner. To me that would keep the lines full with chlorinated water and I have the option to add a sprayer to that return if I needed that to make the water cooler in the summer.