WHERE did 20 million voters go? Something doesn't seem right?

Two things:

First, they haven't finished counting all the votes. That's why several races still haven't been called. There are several millions of votes yet to be counted - perhaps not twenty, but a fair number. Wikipedia says that 87% of the vote has been reported. Not sure how accurate their number is, but there's probably north of 10 million votes that have yet to be reported.

Second, it was always a concern among Democrats that between Harris and Trump, a non-trivial number of their voters would choose the couch. We won't know until the counting's done and the data has been sliced and diced...but it's entirely possible that Democrats just weren't as willing to vote for Harris as they were for Biden in 2020.

I think one main throughline of this election is that the electorate was really unhappy with Biden's performance in office, and that his historically low approval ratings were not something that Harris or the national party were going to easily shake.