In a celebratory tweet posted to the Twitter account of Jill Biden, president-elect Joe Biden wore a hat that was maybe (probably) a trolling response to the hat often donned by the still-for-now-president and some of his supporters. If that sounds stupid then it's just because I really don't know how to put that in a way that doesn't sound stupid.
Anyways, the probable-rebuttal-hat (make your own judgment, but that would seem to make the most sense given the context) said "We Just Did. 46." It appears to be a response to the Trump slogan "Make America Great Again."
Some people noticed said hat and, perhaps after hours of thought, had a Eureka moment. For example:
On a related note Trump has yet to concede the election, despite all signs pointing to him losing.