Adding Buffs

Hello,
How do you add a buff to a unit?
I have no idea, I looked in the triggers and it only said "Remove Buff",
Could I please get some help here.

It is not so easy. You are gonna have to mess with abilities and dummy casters, for example if you want to add inner fire buff you need to make an invisible unit cast inner fire on your target

Buffs do not give the actual effects of abilities, so it's likely you don't actually need to "add a buff" to a unit anyway.