It can be proven to the extent that any rational and sane man can see it.
A few weeks ago someone posted a link to a youtube video featuring one of the guys who worked in the Arizona audit of Maricopa county Arizona. He was testifying before the Arizona Senate.
He said they handed him the military ballots to process. He said in 2016 there were 1600 military ballots. In 2020 there were 9600, and they were not actual ballots. They were copies of images of ballots on 8x11 printer paper, and they were all for Joe Biden at the rate of 95%.
If you know anything about Arizona's voters, or about Military voters in general, these absentee ballots are false on the face of it. Literally impossible for those numbers to be correct.
2020 was a Stalinesque election.
Were they all really xerox copies? I don't doubt there was cheating, and it's likely that there was enough cheating to sway three states to Biden and give him the election.
Most of the fraudulent ballots in the states in question (and other states) would probably have been real ballots, collected and filled out by Democrat operatives. They wouldn't have looked any different from other ballots, so there would be no way to tell which ballots or how many ballots were fraudulent.
When I said "We don't know who really won," I meant something like the 1960 election. "We" sort of know that Kennedy stole the election, but not everyone will agree with that and it can't be "proven" in a way that would stand up in court, so "we" as a country will be arguing about this until the end of America.