Like, if a Nickit used Beat Up, would each hit get STAB? Or would at least its hit get STAB?
From Generation V onwards, yes.
Beat Up now receives STAB from Dark-type users and no longer deals typeless damage.
Beat Up (Bulbapedia)
Hope I helped!
“It will also receive any boosts from the originating user, such as STAB and any applicable items” So if the Pokémon using beat up is dark, all hits will be STAB
Source: https://pokemondb.net/move/beat-up
Instead of using each party member's Attack stat, the user's Attack alone is used to calculate the damage of each strike. Because of this, Beat Up is now affected by boosts like from Swords Dance and Attack-boosting items like Choice Band. Furthermore, the base power per strike is no longer 10, but instead individually based on the Attack base stats of the party Pokémon:
Base power = (party member's base Attack) / 10 + 5
Beat Up now receives STAB from Dark-type users and no longer deals typeless damage. Beat Up no longer lists the user's name. Each strike will activate Justified and Weak Armor. If a Focus Sash, Focus Band, or Sturdy activates before the last strike is dealt, the following strike will cause the defending Pokémon to faint. A Focus Band can still activate repeatedly to prevent fainting, but each chance is independent. Each strike has an independent chance of activating Stench, but this does not stack with the Razor Fang or King's Rock.
