If you compare the 2021 election and 2025 election in 2 snap shot, this will be it
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2 things you will notice
1.) 2025 election had almost double the turnout than the 2021 election
2.) Mandani won with 50.4% of the vote, which means 49% of the other votes for not Mandani
So we can see the following. If we compare the pre-election poll and the last election result, you can see a similar number of Democrats that does not support Mandani voted for the only other Democratic candidate, Cuomo
So if this is 2021, let's say Mandani does not happen, Cuomo would have replicated the result Adam did (considering some GOP voters who supported Silwa in the pre-poll switched to Cuomo during the election, that's the reason why Silwa only had 7% vote.) Which shows you the base for the Democratic establishment remains unchanged, that literally the same number of people (well, maybe dropping 2 or 3 points in support) that had voted for Adams in 2021 voted for Cuomo
On the other hand, Mandani happened, and this jumped the turnout by almost 100%, which means the sheer volume of Mandani's vote that had not been covered before (assuming new democrats' vote is not going toward Cuomo, which is a reasonable assumption to make) won him this election.
This is extremely similar to Trump's MAGA and generic GOP vote in some sense, without Trump, GOP Ballot suffer a down turn and Trump involvement give any election a 3 to 5 % boost, that's how he taken the Presdiency from the Dem in 2024, that's also why most poll got his vote wrong, Mandani effectively was doing the same thing, his present in a ballot box increase the turn out, that's is the key as to why he was elected, I don't think most Dem establishment actually supported him in this election, but he won it by basically putting himself out there, be the talk of the town, and that's how he won it.