From: Vienna, Austria
The base values used are the anti armor/anti soft values - the firepower - of the firing squads vs. the def/armor alue of the defending squad.
This is modified b a lot of different variables, like forts, terrain, supply, fatigue, exp, weather, commander land skill,...
The outcome of the combat rolls decide whether a squad is hit, and damaged or destroyed.
It is important to know that AV and firepower are only related to earch other as both increase with number of guns. There the relation ends.
A 300 AV US Div in '44 has seriousely more firepower than the same division in '42.
Itdepends: you don't need to damage a device first to destroy it. You are maybe relating to this: if more than 50% of the devices in a unit
are damaged, there is a random chance they will be destroyed without combat at turns end.
Hope that helps.