Its a sandbox game just like Arcage, Neverwinter Nights, Albion Online, Everquest, Star Wars Galaxy, Eve Online, Black Desert Online, etc in which the game's goals tacitly produce a local society, typically from players' in-game avatar "Characters" just playing the game. PvP is emphasized on many of the servers.
Right now there is a very good UO Freeshard server called "UO Outlands" with hybrid rules that are quite satisfying and with updated maps/dungeons/landscapes designed to encourage the classic vibes you've heard about and aren't really available in other, modern games.
If you just want to take a look, there is a Youtuber named "Trammie", he prefers to play a pure-thief character and he semi-regularly records streams of live play. Its a good look at one high-level play style that is nearly unique to UO and still available.