Arma 3 is a tactical large scale shooter with a variety of in game items (jets, choppers, boats, submarine, tanks, armored vehicles, trucks, quads), it also has a variety of mods that completely change the game, some of gamemodes that might or might not require a mod installation: (Dayz mods, Wasteland, Battleroyale, WW2 mod, Vietnam mod, and lot more that I dont know!)
A big community that creates mods for Arma is called Custom Combat Gaming, you can check what they offer for Arma3 mods, also to check Arma3 gameplay or arma3 content (sometimes scripted scenario) I can only recommend FrankieOnPc1080
Dayz Playlist
https://www.youtube.com/playlist?list=PLuMTZBpxpB0cB3p3ewQT3KTdZ6szpBYSZ
Arma Adventure
https://www.youtube.com/playlist?list=PLuMTZBpxpB0db2gmFnVVEZeINaO-b2Gc6
For the technical part Arma 2 and 3 are known to be CPU intensive games and they are also known to have FPS over network issues, so online gameplay for normal community servers hosting mods will be running 30fps-45fps not because your PC cant handle it but because optimization issues (CPU and GPU usage are 50% or below), Offline gameplay (campaign, mission) does not suffer from this issue, even so the game is still enjoyable, I still play it from time to time when I'm in the mood