I want to know about how many people are living in places where they are represented by people of the other party. I know there were a lot of crossover districts, but that is not the point. In other words I want to know how representative the House of Representatives is based on presidential results.
There are resources breaking down the votes by congressional district for president.
Per this metric, Pennsylvania's 3rd district, where 9/10 voters voted for Joe Biden, is the most representative. On the other hand, the least representative is New York's 24th district, where a Republican is representing it despite Trump getting only about 45% of the two-party vote.