Hi and welcome! Please don't confuse ignorance of a VERY complicated pool set-up with dumbness. It's not. I must admit, I don't know why there are two outlets on your skimmer, either. At a guess, since it's a big one, the installer can use either one and leave the other closed...but that's just a guess.
"Popups" usually refer to a self-vacuuming system where, in the pool, heads like sprinkler heads pop up to vacuum up all the dirt in the vicinity. We ofter hear "several popups don't work" and generally suggest other automated vacuum systems.
As for all the plumbing and valves, it's kind of hard to know what everything does. Our site owner, PoolDoc, aka Ben, and some other here are pretty good with unraveling mysteries such as yours.
You'll also want to learn how pool chemistry and management of your water works. It's really not difficult, though many in the business try to convince you it's a black art. It isn't. In any case we have tried and true proven methods for easy, simple and inexpensive pool care that doesn't use and "magic" chemicals and formulae--just simple clear stuff.
For example, you can chlorinate your pool just fine with ordinary household bleach. In fact, it's one of the best sources of chlorine for your pool. Many chemicals are simple grocery store products sold at far higher per-pound costs in pool stores when they are re-labled.
We have lots of info on it here, in our stickied threads, and on our sister site, poolsolutions.com.
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