I am a bit confused on the resolution of tanks firing on tanks. 20.6 states that the attack number is determined by subtracting the targets armor value from the attackers FP which results in a negative number, which confuses me on how they could ever be destroyed, even with modifiers.
AFAIK, there is no concept of "1 = automatic hit" for tank vs tank (unlike infantry attacks), so indeed a negative number means no chance of success. (If I'm wrong, someone please correct me!)
I assume that means that the attacker's firepower is simple physically too weak against the defender's strong armor.
But note that often the defender's side/rear armor is sufficiently weaker, so that the same attacker does have a chance in a non-frontal attack.
And of course infantry have 2 different ways of attacking tanks, so tanks are not necessarily invulnerable even if your own tanks are weak against an enemy's well-armored tanks.
I also found tanks close to useless against infantry which are in any terrain/ buildings/ foxholes.
That depends on the specific tank. Some have excellent strong anti-infantry weaponry (e.g. strength 10 or 11 vs infantry!), and some don't!