There is Arma2 and there is Operation Arrowhead. These are stand alone games. Usually after installation they would appear as 2 separate icons on your desktop.
If you merge them into one game you get “Combined Operations" ("CO") and only one icon on your desktop.
Combined Operations gives you access to all the maps, soldiers and equipment used in both games - all in one place.
Subsequently there were add-ons that are well worth while. BAF, PMC and ACR. These too are merged into “CO” so you still have only one game.
Secondly there is Arma 3. It has many improvements over Arma 2. It is a separate and stand alone game with one icon on your desktop.
Thirdly there is Iron Front. This has one add-on of US forces and the Omaha Beach map. This add on is well worth while. Iron Front is a separate and stand alone game with one icon on your desktop.
The Iron Front developer released 5 patches after release and you should patch the game.
More recently fans have created a way to merge Iron Front with Combined Operations. Some say this has been successful but some have found problems.
More recently again, fans have created a way to merge Iron Front with Arma 3. Some say this has been successful but it seems there are many problems so it is probably not worth your trouble for the time being.
To generalise, Iron Front is built on the Operation Arrowhead engine. My suggestion is to Install Combined Operations, Iron Front and Arma 3.
You say you have never played the series. To learn the game I found the best tutorial is the one in Operation Arrowhead. Then use the section called “Armoury”. Many types of missions are given to you at random. Go back and forth between the armoury and the tutorial till you have mastered it. In my case this took 6 months!
Then use those skills to Play Iron Front.
Arma 3 is apparently somewhat different. It seems experienced CO players can pick it up easily. I have not played it so I cannot say for sure.
Mostly, have fun!