Z-Abilities

I dont get the question

Assuming they have the proper tags, your 3 bench units' Z abilities benefit your 3 active fighters as well.

All 6 character's z-abilities are active even when they're not in battle. you want to pick what are referred as 'bench units' that have z-abilities that buff the stats of the 3 you take into battle. That's generally what a good team is built around and is called 'synergy'.
tl;dr yes, the "Z-Ab boost all fighters the ability is applicable to