I don't understand how GTA Online works, is it just doing normal missions but with other people on the map?
It is a completely separate game to the story. Only the locations are the same.
There are so many different facets to GTAO to mention here.
It can be played solo, with friends, with crew mates or in open play.
A lot of the content needs to be played with others and in open play.
Original 5 Heists need a minimum of 4 players and are story based mini campaigns in co-op. The New heist released, Doomsday heist, can be done with 2 to 4 players and takes about 6 hours from start to finish with ta good group.
It is worth a look as there is so much there, however there are also drawbacks. Whilst the community is quite good, in game players can be quite toxic and griefing.
I am not a PVP player and manage just fine with my playing group.