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As @ryanabest's cool interactives show, there was actually very little split-ticket voting this year: https://fivethirtyeight.com/features/there-wasnt-that-much-split-ticket-voting-in-2020/
Were there a handful Biden voters who voted for Republicans for Congress to put a check on his administration? Sure. But there were just as many Trump/Democratic voters! https://fivethirtyeight.com/features/there-wasnt-that-much-split-ticket-voting-in-2020/
The real reason for the apparent disconnect between presidential & congressional results? The same votes produce different results because of each election's different rules & playing fields. https://fivethirtyeight.com/features/there-wasnt-that-much-split-ticket-voting-in-2020/
The same presidential votes that gave Biden 306 electoral votes, if applied to these Senate and House maps, would have given Democrats 49 Senate seats (with GA runoffs still TBD) and ~223 House seats—basically exactly what they actually got.
