In addition to the excellent answer already provided, I would like to point out that you are interpreting the phrase wrong. The phrase is not "blaring YMCA — the song".
The dash (—) in this case is used as a separator to separate an explanation from the thing being explained. The dash is used as a replacement for parentheses here.
The thing being explained is "YMCA", and the explanation is the entire following subclause: "the song President Trump closed out his latest rallies with". So, Kaitlan Collins is explaining here for readers not familiar with the song or its usage, especially by the Trump Campaign, that the Joe Biden supporters are playing exactly the song that Donald Trump used in his campaign rallies. In other words, the Joe Biden supporters are mocking Donald Trump by playing his own campaign song back to him, after he has lost the election.
An alternate way of writing this sentence with parentheses instead of the dash would be something like this:
The crowd outside the White House celebrating Joe Biden’s projected victory is blaring YMCA (the song President Trump closed out his latest rallies with).
As a subclause:
The crowd outside the White House celebrating Joe Biden’s projected victory is blaring YMCA, which is the song President Trump closed out his latest rallies with.
Or, as a separate sentence:
The crowd outside the White House celebrating Joe Biden’s projected victory is blaring YMCA. YMCA is the song President Trump closed out his latest rallies with.
Replacing parentheses with a dash is a common usage of dashes. A dash can also be used as an interruptor or to signal a pause. In this case, I feel that in addition to separating the explanation, it also serves as a pause. If I were reading this tweet out loud, I would indeed add a pause for dramatic effect at that exact place:
The crowd outside the White House celebrating Joe Biden’s projected victory is blaring YMCA [PAUSE] the song President Trump closed out his latest rallies with.