They can retreat to the bench, be knocked out, be equipped with a tool, and attack once per turn, just like anything else.
Once out, those four cards are treated as one entity, and work just like any other Pokemon would. And it must be all four corners, four copies of the same part can't make a V-UNION. 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. They're not considered Basic or Evolved Pokemon, and exist as their own thing, meaning there is no way to put them into play from your hand. V-UNION Pokemon work differently from normal Pokemon cards.