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The earth, depopulated except for Noah and his family by the flood, was repopulated by Noah’s descendents.
Genesis 7:23 And every living substance was destroyed which was upon the face of the ground, both man, and cattle, and the creeping things, and the fowl of the heaven; and they were destroyed from the earth: and Noah only remained alive, and they that were with him in the ark.
Genesis 9:1 And God blessed Noah and his sons, and said unto them, Be fruitful, and multiply, and replenish the earth.
It is described in Genesis 10.
Genesis 10:1 Now these are the generations of the sons of Noah, Shem, Ham, and Japheth: and unto them were sons born after the flood.
32 These are the families of the sons of Noah, after their generations, in their nations: and by these were the nations divided in the earth after the flood.
A count of the names will yield 70 families into which the earth was divided when their languages were confounded at Babel as described in Genesis 11.
By these was the earth repopulated.
Acts 17:26 And hath made of one blood all nations of men for to dwell on all the face of the earth, and hath determined the times before appointed, and the bounds of their habitation;
That the flood actually happened is confirmed by
1 Peter 3:20... when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water.
21 The like figure whereunto even baptism doth also now save us (not the putting away of the filth of the flesh, but the answer of a good conscience toward God,) by the resurrection of Jesus Christ:
Matthew 24:38 For as in the days that were before the flood they were eating and drinking, marrying and giving in marriage, until the day that Noe entered into the ark,
39 And knew not until the flood came, and took them all away; so shall also the coming of the Son of man be.
The writer to the Hebrews
Hebrews 11:7 By faith Noah, being warned of God of things not seen as yet, moved with fear, prepared an ark to the saving of his house; by the which he condemned the world, and became heir of the righteousness which is by faith.
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I hope this helps.