At the national level it a tax credit versus a tax rebate. So in order to get the full $7500, the buyer needs a tax liability of at least $7500.
In Ontario and other Canadian provinces that have/had it, it is/was a straight up rebate on the price of the car. Money in the purchaser’s pocket, essentially. Almost all of these rebates went to people buying Teslas, although some would have gone to purchasers of Nissan Leafs, Chevy Bolts, and Kia Soul EVs. But even those mostly appeal to well-heeled buyers who can afford to lay out that kind of money for what is most likely a second (or third) car.