Anyway, back with your cookie and mum thing:

with cookie, we do use the word here but it’s got a more specific definition. A cookie here is a large biscuit with chocolate chips in - subways cookies for example we still call cookies. But stuff like an Oreo is a biscuit.

With mum, think this is on accents. Since a lot of Americans I’ve spoken to have said that “mom” is with an “o” sound. For British people though, like me, it’s sounds like a U, so both spellings I find reasonable.