They asked Mamdani if he thought Trump was a Fascist and he hesitated, then Trump said "It's ok, you can just say it, I don't mind" and pats him on the back and he says yes.
Discussion
The thing they have in common is that they're both popularists that ran on anti establishment campaigns.
Trump benefits by positioning himself with Mamdani, Trump's base are brain-dead and will vote Trump no matter what, but just seeming to be on good terms and working together helps Trump's image with younger people.
And Mamdani benefits by not getting all the state funding cut etc.
One of so many things they both agree on!