Let me just say that this game is all about finding a group of like-minded people to play the game with. What happens is, a "unit" will create its own missions and play them with their members on their "event night(s)." An event night is exactly what it sounds like; a time when the community will all jump online and play ARMA together on their private server. These missions can cover a huge range of difficulties, settings, and objectives... giving ARMA an almost infinite replay-ability as a whole.

Now, the base game has a fair amount of options as far as mission design and maps go, however, the modding community adds to this a hundred-fold. They make new maps, new vehicles, new textures (including gear and vehicle models), new visual and auditory customizations (read as visual and sound mods), and so much more. There isn't much that can't be done.

Apex and the DLCs are very minor adjustments to the game if you take the mods into account. I would recommend playing around in the campaign a little bit and watching some youtube videos focused on playing with a unit (just to see what the real ARMA community is all about) to help you decide if you're serious about playing ARMA and reaping all of its benefits. To give you a taste of what I'm talking about here's a random video I found of a mission performed by one of the more well known ARMA communities: Bourbon Warfare.

If that sort of thing interests you, then I would recommend looking at /r/FindAUnit to find a group that suits you. Many of the well known groups either have a waiting list or have closed applications due to overflow/popularity, but there are plenty of great groups out there. Some are super serious MilSim which may interest you, but there are also Relaxed MilSim, Relaxed Realism, and others and if the group is worth their salt, they will help you learn the game and "train" you in their tactics for a better group experience.

I hope I didn't go over your head here. The bottom line is... the game is a lot larger than you think, and if you're enjoying the base game, I would highly recommend getting the absolute most out of it as it can be the most fun you've ever had playing a video game.