It plays like a Fallout or Elder Scrolls singleplayer RP game but with other players on your map. It’s not really comparable to any MMO. There are quests and you’ll increase in level and strength with added perks. Some areas are more dangerous than others. PVP and the environment scale (to a degree) to your level. Groups can consist of four players.
The limited server population is for immersion. This is the wasteland; hordes of players running around would kill that vibe. They intentionally created no main towns so that players wouldn’t congregate in a certain area. Though they expect that players will create their own “towns” or hubs.
There are no NPCs. You get quests through holotapes and other lore. The players will become the factions. So you can be a raider, Brotherhood of Steel, etc.
Bethesda has said that they will be implementing private servers later. So OTG could have their own servers in the future.
Private servers will be able to be modded and restrictions can be placed on PVP. I’m not sure if private servers will have a population limit. There’s not much info on this yet; Bethesda said more info will come after launch. Factions are also not implemented fully yet, though you’ll find quests to become a part of them.
As far as end game, who knows. The game map is large and only encompasses West Virginia, so I speculate that other states will be added later. The design has definitely allowed for further growth. I anticipate seeing more PVP modes and expansions to the construction aspect. Currently endgame seems to be about controlling the few nuclear silos (large scale PVP), which follows the storyline.
Players will likely create towns for trading and role-playing factions. You can’t remodel or add to the existing landscape, but you can build camps and share campsites. The campsites will despawn if another player places one nearby when you are logged out. You can blueprint all your camps and workshops though, and place any of them when you are out scavenging. The game also saves progress when you spawn a camp, so it’s encouraged when you are going to head into dangerous regions. You can fast travel to any player, friend, or camp on the map.
Check out the other posts in this section. I haven’t played since beta and there were changes with the first patch.
There are links in this section to official information and other members might have some intel. I posted a thread with guides here.
Welcome to the wasteland! 
(Edit: sheesh, I didn’t realize how long this was lol - enjoy my novella)