That's 100% not how it works. In a court, the onus is on the plaintiff to prove something happened. Thus, the onus is on Trump's team to prove fraud did happen.
The police don't go to your house and say "prove you didn't murder someone". They instead have to prove you did murder someone.
If I claim that you fuck a horse, the onus is on me to claim you did fuck a horse. Not on you to prove that you didn't fuck a horse
You seem less mad today, little guy. Did your mommy remember to get honey mustard for your tendies?