ACAT – All Creatures Are Traitors – an April Fool's mod from 2022.

comrades, please tell me how to change unit stats and give them abilities?!

as an example, take this mod - it only changes characteristics, so there's nothing extra in it

all original characteristics are located in data.pak along the same paths as in the mod, so finding them won't be difficult

with characteristics everything is simple 
<AttackSkill>4</AttackSkill> - attack 4
<Health>10</Health> - 10 health
and so on

but with abilities it's harder
for example
<Abilities>
        <Item>ABILITY_ELVES_DOUBLE_SHOT</Item>
    </Abilities>
these are the original abilities of wood elves - one ability (double shot)

and here are the abilities from the mod
<Abilities>
        <Item>ABILITY_ELVES_DOUBLE_SHOT</Item>
        <Item>ABILITY_TREACHERY</Item>
    </Abilities>
the treachery ability was added

I'll say right away, I don't have a list of abilities, I just look for them in the characteristics of other creatures, sometimes it's hard to distinguish creatures, for example who is a demon and who is an imp (they're called imps and familiars), then I scroll down to the end of the file and check if there's anything in the <Upgrades> section - if there is, then it's an unupgraded unit