For boarding you end up boarding at the same time as pretty much all of United's elites, other than the GS and 1Ks. So yeah, for United hubs there will be a number of GS and 1Ks, but you're boarding alongside the rest of UA elites, not after them like your first post made it sound like.
On a separate note, as others have said, upgrades aren't part of the Star program. However, a couple weeks ago I did end up getting upgraded on a UA flight, at check-in nonetheless. I know there were other UA elites on the upgrade list (I've taken this flight a couple of times and its always had 30+ people on the upgrade list. This flight was no different). I can only surmise that somehow I lost my seat assignment (I'm not sure I actually ever had one) and when I went to checkin the night before the only seat left was one in first class and for some reason I ended up with it instead of a UA elite. I'm fairly certain I was on a US codeshare and checked in via the US website, which adds another level of complexity in the 'how the heck did that happen' category.