[old_tv_nut]

Ok, so you now have the idea that you need to keep in mind whether you are discussing time domain or frequency domain.

So:
"What I don't get is how brightness corresponds to the sample position (or in case of audio, how loudness corresponds to audio frequency)??? If the carrier is modulated by frequency, it is moved up and down, but the sideband that describes the position of a sample, is relative to the carrier, how does it work???"

You should now understand that loudness does not correspond to audio frequency, but to audio amplitude. And that a particular video sideband does not describe the position of one sample, but a repeating pattern of light and dark.