Where did Moses and Elijah come from at the Transfiguration?

If the gates of Heaven were closed after Adam & Eve sinned, where were Moses & Elijah when Jesus was conversing with them during the Transfiguration on Mount Thabor?

After his death Moses likely went to the abode of the dead – the same place to which Jesus descended after his death, before the Resurrection. Elijah, however, did not die but, rather, God made an exception for him and took him to heaven.

See these related posts:

Did everyone who died before Jesus go to hell?
Did anyone go to heaven before the Resurrection?

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