Is it better to buy a used car from that specific brand’s dealership?

No, it doesn't make any difference,

*Unless* you're buying a Certified Pre-Owned.

In order to qualify for CPO, OEMs require dealers to recondition their used cars to a very specific standard.

And every OEM holds its franchisees to a different standard.

Regardless, there may still be no difference between buying a CPO

Vs

buying the same car from a reputable used dealership.

I know many used car dealerships that recondition all of their cars to CPO standards.

If not higher.