This is something you'd learn in your first year at law school. If, as Trump's complaint alleges, Dominion did destroy evidence then the evidence is immediately assumed to support the opposing party's allegations.
Biden's defense would now have to prove there was no fraud as if Trump's team had certain evidence of there being such.
Once a plaintiff has clearly demonstrated that its case is founded on clear and convincing evidence, the burden of proof to explain shifts to the defendant.