Jehovah caused the flood to remove all flesh and wickedness. If the Nephilim built the pyramids then the pyramids were built preflood as we know only 8 people survived the flood.
As for extraterrestrials other then angels, cherubs and seraphs. .. Years ago when I was learning the truth about our Father, I remember reading the reasoning regarding beings on other planets. It brought the answer home to me. I found the Awake article and have copied it in part the following.
"If God did create such beings, he did so before he created Adam and Eve. Such beings either remained faithful to their Creator, or like Adam and Eve, they sinned and fell into imperfection. But if they became imperfect, they needed a redeemer. As one essayist put it: “One has this dreadful thought that on Friday [the day Jesus Christ was executed], every Friday, somewhere in the universe Jesus is being hanged high for someone’s sins.” But that is not Scriptural. The Bible tells us that Jesus “died with reference to sin once for all time.”—Romans 6:10. What if these beings had remained perfect? Well, when Adam and Eve sinned, they were, in effect, questioning God’s right to rule over a world of intelligent physical beings. If another planet existed at that time, a world full of intelligent physical beings who were living harmoniously and loyally under God’s rule, would they not have been called in as witnesses to testify that God’s rule does indeed work? This conclusion seems inescapable, since he has already used even imperfect humans as witnesses in his behalf on that very issue.—Isaiah 43:10."
. I had forgotten the reasoning and am so glad to have experienced it again. Praise Jehovah