how does Bonus Mod actually add the bonus? (not a math question)

ok i think im missing something in bonusmod, what mothod does it use to add/remove bonuses? does it add multiple passive abilities to the hero? and if thats the case why don't they appear in the units UI? i searched the forums for answer but non that simply answered that question, thx

It adds the item abilities (item life/mana/armor/damage bonus) to the unit. These won't show up in the units UI.