Remember that Trump, with almost no exceptions, NEVER was ahead against Joe Biden in ANY poll during the 2020 Presidential Election.
https://en.wikipedia.org/wiki/Nationwide_opinion_polling_for_the_2020_United_States_presidential_election
There was essentially clear unanimity among all polls during the entirety of the 2020 Presidential race that Joe Biden would handily beat Trump, which he did.
What the Big Lie is premised on is solely the "Red Mirage" on Election Night. In other words, many states released Election Day voting results showing Trump ahead, only because R voters tend to vote on Election Day. In fact, Trump bent over backwards to tell his voters to ONLY vote on Election Day and not to use mail-in or other early voting means.
Once states started counting mail-in and other early voting, Joe Biden gained heavily all over the US and then surpassed Trump in some key swing states, which ended the fiction of the Red Mirage. This is called the "Blue Shift" and is/was entirely expected. Please search "Blue shift politics" in Wikipedia for a good description, including in the specific context of the 2020 Presidential Election...for some reason, I can't get the link to post properly here.)
Trump has used the Red Mirage to perpetuate the lie that he had won on Election Night. No two ways about it. He tried to couch the Blue Shift as voter fraud, but that was debunked by everyone, including every court that had cases about voter fraud/irregularities.
Never forget that.