This is probably too complicated for you but I'm going to give it a shot, please read slowly and really try to follow along:
1. The last democrat that run as an opposition candidate prior to Biden was Obama in 2008. He got over 69M votes
2. When Biden ran as an incumbent during the COVID election which brought historical engagement and turn out he got 81M votes
69/81 = .85 or a 15% increase (That was Biden COVID jump
I love it. Every time one of you chimes in with that answer it just reenforces my OP. Thank you