Well, first of all populism isn’t really a defined concept that everyone can agree with. Because of the question you’re asking I’m assuming you’re talking about populism as represented by such politicians as Trump/Milei/Bolsonaro/etc.

In that case the answer is that Populism is not fascism in the same way that elections are not democracy. As in, you can have one and not the other, but they tend to go hand in hand often enough to be very strongly associated.

Sure, you can have a democratic populist, but in history most examples of populism were preludes to fascist. And a fascist government usually uses populism to keep some sort of approval.