The only way to get a V-UNION Pokemon out is to assemble all four cards in your discard pile, where they can then enter play as one combined piece. And it must be all four corners, four copies of the same part can't make a V-UNION. Once out, those four cards are treated as one entity, and work just like any other Pokemon would.

A V-UNION Pokemon can only be played to your Bench when all four cards are in your discard pile. Since all four cards of a V-UNION share the same name, you can only have one of each V-UNION Pokemon in your deck. When fully assembled, they’re neither Basic Pokemon nor Evolution Pokemon — their stage is “V-UNION.”