You can go solo for everything, all proper missions take place in a somewhat confined area (like inside a building, still pretty big), if you want you can make a group or queue for one yourself for every single mission at the entrance right before starting one, as well as choosing difficulty, but there’s two or three missions that include a particular mob type called heavy, they are walking sponge gatling gun turrets which only weak spot is on the back and they always face towards you, it takes a lot of effort to kill one on your own and one mission trows you two of them at the same time.
Then there’s safehouses across the map which are control points of sorts, they are little rooms(like an apartment or small office, one is a school’s gym) where you can go to rest, restock and buy equipment and you can see other players around, you can team up with individual players if you wish (up to three for a team of four) to free roam the world and take on missions, otherwise you’re all alone.
The Dark Zone is the multiplayer zone, everyone is up on their own, you can team up or go solo, but you’ll see other players going around, friendly fire is a thing, so you have to watch where you shooting at unless you want to be a rouge, then you’ll get a bounty over you or your group that will increase the more players you kill, I once paired up with a dude, he was grinding levels and I was looking for loot, I just started to follow him like “I’m just going in the same direction don’t mind me”, we never team up properly but we had a good time until we got destroyed by a group of rouges.
Incursions are endgame super hard missions, there’s four, but you need the DLC to access two of them, I went in one like “I can do this”, the first group of baddies had three explosive things I needed to blow up a blockade, I kill them, bastards keep respawning, it was obvious you where supposed to team up so someone was distracting them while someone else did the breacher’s work, still can be done solo, someone made a video of it, crazy stuff. The other one requires at some point activating two switches at the same time so you need to team up for sure.
Forgot to add, they added three missions which are pretty much horde modes, they are doable in the lowest difficulty but you do need a team for higher rewards, unless you get insanely good at the game.
This is just the base game, DLC adds more to it but I’ve yet to buy that stuff.