if i use retreat and it resolves. does the combat end right away or can the opponent still use an action as if it was still ongoing?

No — combat doesn't end just because Retreat resolved. The showdown stays active and Focus keeps passing, so your opponent can still play actions (Rules 346, 348).

Once Retreat resolves and the chain empties, the state returns to Open and Focus passes to the next player in turn order. That player gets to play spells/abilities with Action or Reaction if they want — Retreating your last unit doesn't skip that. Combat only closes when both players pass Focus in succession without starting a chain, even if no units remain at the battlefield (FAQ #1229, #976).