Majority-minority counties is what they're doing. They deliberately redraw the lines so that most of their counties are mostly comprised of minorities, who tend to vote to the left.
Or, in other words, racially-biased gerrymandering.
Fortunately, the SCOTUS is supposedly getting ready to strike this down (Louisiana v. Callais).
That poses a major problem for the Democrats: if the SCOTUS rules against them here, for obvious reasons, it will sink them. All of those racially-gerrymandered blue counties will suddenly be in jeopardy as they are no longer occupied almost solely by minority populations. And with that, the House is likely to become solid red — and it may remain that way for a very long time. And it just so happens that right now, the chances of them declaring it unconstitutional appear to be very high.