You complete a dungeon when the ability on the bottom room resolves
If your turn has multiple combat phases, Ursine Monstrosity's last ability triggers at the beginning of each of them
Doing so lets you search your library for a Dragon creature card and put it directly onto the battlefield, which is a ridiculously powerful ability given all the wild Dragon cards Magic has printed over the years
As for what didn't quite make the cut in the full version, there's been no shortage of these effects pasted onto permanents: Strixhaven continues to be a never-ending source of new niche goodies, with Lisette, Dean of the RootLisette, Dean of the Root being almost as much of a boon to a lifegain deck as her Witherbloom co-dean
The quality of some of these knock-offs is good, while others are very bad
However, there were a few cards like Brain Control that became powerful tools and King of the Skull Servants that would later turn into a one-turn kill method