Nimrod is Cush's son, not Canaan. Cush would have received the best inheritance and blessing over his brothers, and it obviously blessed Nimrod as well. As the population group came down from the mountains Turkey following the Tigris and Euphrates, we have the first post Flood civilization.

Noah stayed in Sirnak (City of Noah), and the elders apparently allowed governance by one king (Nimrod) until the incident at Babel where God scatters the clans by language barrier. Then being the closest to Babel, Nimrod remains his clan's status and was the first king after the scattering too.

He lived over 500 years old, almost all of Noah's son's and grandsons lived in the hundreds of years. They all still seemed to possess blessed DNA/genetics to adapt well, heal, etc. Neanderthal and Denisovans are splinter groups from the early post-Flood people who became mainly isolated from the rest of the population. From whites in the North (lack of Sun meant less need for melanin) to the really dark Africans (adaptation to neutralize all the sun) and everything in between.

One translation/explanation for the Nephilim is that it is referencing the heroes of old, the legendary people that are 'just stories'. Their legend came from that time when the fall happened and the angels bred with humans. But the Bible says they were all destroyed, and yet also says they were after the flood too. I take the latter to mean due to genetic leakage downstream, more of those legendary type people did exist afterwards - all the way until Joshua and Caleb, etc are hunting them down.

These beings probably lived for hundreds of years and probably kept growing all their life. Giant slayers and all those types of legends, etc about men hunting them down. The last of the giants was said to be King Og and he was at least 13-15ft tall. Goliath was a descendant of the giants but not pure blood hence he was only about 10ft tall.