How effective are stack bonuses.

I know every stack gets a negative or positive bonus based on the ratio of your troops to enemies troops.

but what is this ratio, and how exactly do stack bonuses work?

also, does arb factor into this bonus?

I tested yesterday inf vs bombers. 51 inf vs 20 bombers: all bombers and 17 inf down.
20 inf vs 20 bombers -> all bombers and 18 inf died.

so its wayne

As far as I know, they don't have any effect. I'll look at a stack of units with the general, it'll have 35 defense points (I have the general defense upgrade) I move the general into the stack, and it becomes 36, the one being the general's defense.
So as far as I know, it doesn't work.

Umm, I thought it was the part of the previous battle system but not this one, correct me if I am wrong.

First of all inf have 4 attack 6 defence as default on most strategies while bombers are 6 attack and 6 defence. When a unit attacks it rolls anywhere from 1 to its max possible attack or defence. For example; if you attacked 20 infantry with 20 bombers as you mentioned your bombers rolled between 1-6 since the max attack is 6 and your infantry also rolled from 1-6 since the max defence is 6. Therefore during the roll your infantry rolled higher.