How are the Democrats suppose to hold the judicial branch exactly?
First of all, it's suppose to remain nonpartisan, and even if you held the view that the Supreme Court IS partisan, then the count currently stands with five Republican-appointed judges and four Democrat-appointed judges.
In 50 years, the most a single-term President has ever appointed to the Supreme Court is two. And that's usually in congruence that a leaving judge is replaced by a judge with the same party that appointed them. The only exception to that rule is when an opposition appointed judge is otherwise severely incapacitated or dies.
Which that happened in 2016 and elected Republicans seemed to have lost their minds over it. Failing to realize that if Obama's third appointed judge, Garland, had been allowed a seat on the Supreme Court, Trump's first appointed judge Gorsuch would've likely replaced Ginsburg, not Kennedy. Of course, this would've been unprecedented but Ginsburg is the oldest member of the supreme court and it would've allowed Republicans to "maintain" whatever control they supposedly have over a branch of government that is suppose to remain nonpartisan. Instead, Republicans made up rules to maintain their majority hold and used fear-mongering tactics to motivate moderates and their base to vote for Trump. No matter how much they may have hated him or found him unqualified.
So, even if Biden somehow got two judges appointed, much as Trump did in his first term, the most the Democrats could supposedly overtake the Supreme Court is by one. Still unlikely as the two oldest Supreme Court judges are both Democrat-appointed.