The graph above shows numbers of popular votes in US Presidential elections over the last 50 years.
In 2020 Biden got more votes than Reagan or Obama, two of the most popular presidents in the last 50 years.
Democratic votes went up almost 15 million votes from just under 66 million that Clinton received in 2016 to 81 million in 2020.
Democratic votes then fell again in 2024 by 14 million votes to 67 million.
Republican votes went up 11 million votes from 63 million in 2016 to 74 million in 2020 and dropped back 2 million for 2024.
So 26 million extra voters appeared in 2020 that were not there in 2016.
Just about all of them did not turn out for the Democrats at the following election while Trump appeared to keep most of his extras.
Reference: https://en.wikipedia.org/wiki/United_States_presidential_election