Pool Main drain clog & plumbing question

I am curious how the vacuum port and skimmers are plumbed into the main drain. It appears there is a line from each skimmer down to the main drain, although I am not sure where it connects into the plumbing of the two main drains. I can "jump" the skimmer and draw water at my two main drains Also the vacuum port is plumbed into the same pipe as the main drains back at the pump – is water being drawn from both the vacuum port and main drains simultaneously, or does the vacuum port have some kind of damper that kicks in if the vacuum is plugged in?

I am trying to clear a main drain clog using a drain jet into the vac port, with the valve to the main drain closed at the pump. I would like to know if there is any mechanical obstruction since the vac port is tied into the main drain valve. Prior to encountering the main drain clog, if I set my suction side valve 100% to main drain with the vacuum out, I did not get any pump cavitation. However, with the vacuum plugged in, I got substantial cavitation at 100%. Now, I get cavitation either way. If I try to use the drain jet from the pump basket, the vac port cover opens up and the pressure does not effectively go to the main drain. Please help me understand how my pool might be plumbed.