The most used bidding system is to bid down for axis in Classic and Revised and AA50. Let’s say player A bids 10 for axis in Revised and player B bids 9, then player A says ok, player B gets axis with a 9 bid. you can use the unit bid which you buy units for the bid amount and place the units before the game starts.
Some players uses a restriction of 1 unit pr territory, and other players have no restriction of the number of units placed in a territory. Others use cash only bids, then the extra cash can be used to buy units during the regular buy units sequence.
For Classic the unit bid level is ca. 20-25? for axis, and for Revised 8-10 for axis. In AA50 +NOs the unit bids are ca. 6-9 for allies. For AA42, imo, it is not yet clear which side that is favored.
LL is a system where you divide all attack and defense points by 6, and then roll a die for any hit points which is not perfectly divided by 6. 5 tanks in attack/defense gets 2 automatic hits in LL, then you roll a die for one tank.
6 tanks means 3 automatic hits and no dice are rolled in the first rnd of combat.