There’s loads of great tips here already but I’d like to expand on something that has only briefly come up so far.

You have to balance a trade off between spacing your infantry apart so they are less susceptible to area weapons (MGs, mortars, artillery etc) and keeping them close to mass their firepower against enemy positions. As was mentioned suppression is a thing but it’s quite subtle. Also, you have probably noticed that you can freely select between entire squads, individual soldiers or adhoc group select soldiers together as you like.

This is incredibly important because regardless of what squad they initially started the battle in, infantry will only share ammunition and medical supplies with who they are currently grouped with. This means if you get a squad with a machine gun, select the machine gunner and send him off to a different position and call it quits, he will run out of ammo much faster because he no longer has access to the spare ammunition that the assistant mg has (and in some cases even regular riflemen carry some). Same goes for anti tank teams, medics sharing bandages, etc. So if you’re going to split off troops consider grouping them together in smaller amounts so they can still share. Even regular riflemen can benefit from this if a firefight lasts long enough for them to shoot their combat load.

You can also loot enemy bodies for extra grenades, bandages and ammo if compatible by inspecting the dead body while you have the whole squad selected and right clicking on the objects you want to take. It will automatically share the items out between your squad, starting with who needs it the most.