You get many responses to this.
to learn more about UA upgrades go to
www.united.com > Itineraries & check-in > Upgrades > Types of upgrades
or FT wiki UA Upgrade types

There is the auto-upgrade approach and the certificate approach -- UA uses the later.
UA provides elites e500 certificates based on miles flown that can be used for upgrading and for 1K/GS system-wide & regional upgrades

And yes you may not get upgraded every flight, there are far too many elites for that.