Transfiguration and Holy Saturday

Hi everyone. I'm in RCIA as a candidate and I'm looking forward to the Easter Vigil on Holy Saturday. Today at Mass, we heard about the Transfiguration, and it brought up a question I've never heard before. I've spent some time thinking about Holy Saturday - how this is the day traditionally described as the day that Jesus went into... the underworld? (Purgatory? Hell? Not sure the proper name...) and spread the Gospel to all the righteous who had died to bring them up to heaven.

Well, my question is - doesn't that include Moses and Elijah? Weren't they part of the righteous dead to be brought to heaven by the resurrection of Jesus? If that's the case, how can they be on the mountain in the glory of God conversing with Jesus? If Jesus hadn't died yet, was the conversation a vision that didn't actually happen, or did it? I'm not doubting both stories - it's just that I feel I'm misunderstanding something.