Hour 6 is also minute 30. There’s some arithmetic for you! Six times five, if you like, or six times ten divided by two if you prefer.
Many don’t have numbers at all, so you’ll need to build a good intuition for fractions and converting those to hours and minutes if you want to read them fast. Most of us do it so automatically we don’t notice, but some of that’s plausibly fraction multiplication.
No, the usual case, where the minutes aren't printed on the clock itself, is that you've memorized the positions of :00, :15, :30, and :45, and you report the time by reference to that.
(non-American here, I may be lacking context that makes it obvious)