Not sure where you are getting this but the overpopulation myth was debunked many years ago. See here for a right leaning perspective: https://www.acsh.org/news/2020/08/05/overpopulation-myth-new-study-predicts-population-decline-century-14953

See here for a left leaning perspective: https://www.bbc.com/news/health-53409521

I've read the U.N.'s Climate predictions, and they mostly predict population increasing until about 2050s then declining around 2100.

The problem this presents is we only have about 20 years to get climate solutions in place before the population starts to decline in rich countries. Then we will not have the labor or resources to enact dramatic climate change policies that are needed in U.N.'s Sustainable Development goals. In almost every scenario U.N. needs carbon capture and other expensive technological advances to combat climate change. If population collapses we are in trouble and won't have the resources to tackle climate change. I don't believe Elon Musk's population collapse predictions, but the U.N. research is very compelling.