The only obvious answers I can come up with are cheating at the local level, just not to the same degree or millions of demonicRATS voted for Trump but couldn't bring themselves to vote a straight Republican ticket.
Both are plausible scenarios to me.
Democrats could only cheat in districts they controlled tightly.
In those districts, every registered voter submitted a ballot whether they wanted one or not.
Those ballots voted for Biden and the statewide offices (Senate races)
That is why the GOP did so well in congressional races, and state houses, but did poorly at the state level and national level.