The threaded pipe is a union 1/2 of it is missing / poss find a match at home depot. This is where water goes in and you will need to supply a pump. The t after the gate valve is the water return to sump the top of t stays open.
Yeah, looks like a bermuda. Maybe model 5c but hard to tell from pic. If its a 5c and my memory serves me correct you need a pump that does 900-1100 gph. IMO not worth fixing, unless you have the right size pump laying around and you don't need to buy one.
Old style simmers like that the air goes into the mix after the pump. Most every new style skimmer the air goes in before the pump and a needle, or pin, or mesh wheel inside the pump chops up the air before it goes into skimmer body. Making smaller bubbles which produces more skimmate with less power and heat. Great skimmer for it's day, but not so much with new designs.