I know there is a pressure vacuum breaker here, but what is the rest?

In my area, it is common for PVB to break if incorrectly winterized, but local water code requires these to be installed and inspected annually, with an inspection report filed with the city water authority. I see duct tape on the PVB so this is likely cracked and causing the leak you mentioned in a comment.

I'd hazard a guess that water does come from the house at (2), flows toward (A), (B) and the hose bibb from there. You can verify this by closing (A) and (B) and running the hose bibb to ensure it still works. Run it long enough to be sure you have flowing water and not just clearing the pipe.

With (A) and (B) turned off, you can also test-run anything that you think is attached to them. Once the water in the pipes (1) and (3) is drained, they should no longer run (and if it's a pool and irrigation as suspected, you'd notice lack of pressure even before the lines are fully drained). You mentioned you may have a separate irrigation line so it's possible that one or both of these don't go where you expect. Specifically, if any of (B), (C), (D) are turned off and irrigation still runs, it's not fed from 3.

(B) (C) and (D) may seem redundant as they all feed or cut off the pipe (3). In my system, (B) is located inside the house to cut water off before it flows outside, so the system can be winterized. (C) and (D) are also for winterizing by pumping compressed air through each zone to clear out any water and prevent it from freezing. The shutoffs prevent air from escaping into the house as the lines are pressurized with air.