FWIW, "impression" means "request". Who knows what the user saw.

Google AdSense is pay-per-click (or some algorithm, based on clicks against and the subject matter's value). In the context of adware rubbish, we are probably talking about AdSense. So no, Google doesn't pay per impression.

DoubleClick (part of Google for some time) may still offer an impression-based product. My experience of them a few years ago was that they'd negotiate on anything if you have enough traffic to make it worth their time.