Welcome to the forum xerez99!
Regarding your infantry question:
Infantry hit on a roll of 1 (or 2 if supported by artillery) out of 6, but defend at 2 (defends at 1,2 out of 6)
ftrs attack at 3 (hits on 1,2, or 3 out of 6) but defend at 4 (defends at 1,2,3, or 4 out of 6)
bmbs attack at 4 but defend at 1.
Any unit can be hit during a battle except cruisers and battleships that are bombarding during a amphibious assault and also subs (but that’s a little complicated and depends on the situation).
So infantry can attack and take out ftrs and bmbs but it depends on the battle and the odds.
I would suggest if you are using ftrs and bmbs for attack and defense, use or leave enough infantry to absorb hits as you mentioned in your post.
Ukr has 2 Russian inf, you as Germany want to take it back lightly as W Rus and Cau is heavily defended, if you hit it heavy, all your German units will die when Russia counterattacks so you would send at least 3 inf, ftrs and bmbs to take it back.
Worst case scenario is that Russia will get two hits and you lose 2 inf but still have a chance to take back Ukr (assuming you had two hits for Germany on round 1, if not Germany should withdraw as there is a chance that Germany might lose ftrs or bmbs).
AA is a war game but also a game of economics, you can’t be losing ftrs and bmbs while the enemy loses inf…
The AA42 map is too small but it’s still playable but you need to be careful where the units are especially Europe and Atlantic.