Metro Card Question

Hiii, i haven’t been able to successfully answer my question, and im just wondering. I’m a the orange Q fare card. I haven’t used it yet, but when I scan it when boarding the bus, how much will they know how to take away from my balance? From what I know, once I scan it, it’s valid for three hours, so do I need to scan my card again somewhere before riding the metro rail? Or how would I show the ticket checkers that I scanned my card on the bus? Hope this makes sense and TIA!