Most units will provide a .html file in their Discord somewhere. That's a modlist, it contains all the mods they use. Download it, then:

Import
Open your launcher, click "Mods" on the left.
Click "more" then "Import a list of mods from a file"
You'll get a popup asking if you wanna subscribe to the mods, select "subscribe"

Teamspeak is used for proximity chat and radios. You'll use either ACRE or TFAR. Ask your recruiter which your unit uses.
Teamspeak download
Download Teamspeak from here. MAKE SURE YOU DOWNLOAD TS3 CLIENT, NOT TS5.
https://www.teamspeak.com/en/downloads/#ts3client
Install it the same way you would any other program.

ACRE2
Launch the game with Teamspeak closed, the plugin will be automatically installed.
TFAR
bully your unit lead into switching to acre Follow this guide.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=860874056&searchtext=tfar+install

Make sure whichever plugin you use is enabled in Teamspeak.

Ace does a lot of things, the only think I'll cover here is the interaction. Ace includes a medical system too, your unit will likely train you on that (and whatever other mods they have)

To Ace interact, hold down Windows key (I suggest rebinding this, I have it on E)
You'll see a circle in the center of your screen, intractable objects will have a white circle. Put the red circle over the white one and release when you want to select something.

For some things you'll need to self interact, it works pretty much the same, but your camera and the menu for it stays in place. Key for this is shift+windows key (I have it set to Q)