Geometric information within 1D streams of electrons/photons
Time is the dimension you are missing!
At the end of the day the video signal amplitude at any given moment represents brightness (and colour) at one point, but that point scans across quickly and down the screen slowly.
Both the phosphors decay time and the eyes flicker fusion frequency combine to give the impression of a complete image.
The geometry is encoded in the sync pulses (Negative going pulses that drop below the normal black level) that are detected to reset the scan circuits to the start of a line or top of the screen.