God does all. People lived really long back then, until they started getting naughty, so God said “Hey, you guys, I’m cutting your lifespan down to 120, so then you may be less evil.” Basically.
If you would just read the bible you will have answers lol,
And the Lord said, “My Spirit shall not strive with man forever, for he is indeed flesh; yet his days shall be one hundred and twenty years.”
Genesis 6:3 NKJV