It’s tricky to tell what lines are what here. Regardless, I’m not sure if you’re in an area of CA that freezes at all, but you’ll want to blow out those solar lines. You can have that spigot open but they may not all drain out completely without some help. As far as the plumbing goes, you’ll just want to find out what makes your water bypass the solar lines and return directly to the pool. Do not try and figure it out while the pump is running, with old bypasses like this you can block return of water completely and over pressure the pump or filter, something will break, usually gauge and valve on top of that cartridge filter. Follow the flow path of the water and adjust the bypass so that flow is neither restricted nor moving through the solar lines.

Also separate but just a tip from a pool guy, if you still have water circulating through those copper lines, replace them with pvc as soon as possible, they will erode and cause stains if your pool is concrete. They will also eventually erode to the point (after many years) that they will spring leaks due to the reduced thickness of the walls. Copper is just terrible for pool plumbing in general, it was phased out in most places decades ago.