Almost all of the player base on GTA 5 now are online players and has been that way for years now. You start with no money and do contracts, VIP work, races etc to earn money and unlock heists. Buy cars, apartments, aircraft, properties, businesses etc then run said businesses. After you unlock heists you can then do setup missions for the heists and then do the main heist missions. Robbing banks, casinos, cartel houses etc. Typically needs 4 players for the heists and they can take a few hours to do each one. You can matchmake with randoms to do them if needed.
TBH, if you’ve never tried it properly, it is probably too late by now. Every lobby you go in will have high ranked players with lots of money and you will probably die a lot. You can do solo lobbies and friends only lobbies though, to limit the deaths

For PC players there’s also custom servers, but you need to abide by the server rules otherwise you’ll just get banned. In these servers the game is pretty heavily modded, so you can get things like speed cameras etc so if you’re speeding a player who is pretending to be a cop will literally pull you over and tell you off

. With the custom servers you can have real cars, with real engine noises, different soundtracks, upgraded graphics etc. I’ve never actually joined a RolePlay server before so don’t really know what else changes but you get the gist. You don’t get this on consoles, only PC.
Rockstar have hired some of these server creators to work on 6, so that’s obviously the direction they’ll be taking with this game too. The online community is huge, it’s regularly been the most watched gaming topic for the past few years on twitch.