>If, let's say, bank has a policy that you need 20% down payment to qualify for a mortgage, and let's say this has a differential impact on some group, how do you go about addressing this?

You look to see why that's the case, and if it happens to be that those people have been recently (relative to society) emancipated slaves who've barely had 4 generations in a society that still systemically kills them via law enforcement, then you do whatever you can to change that.