C is for battery check. If the battery is good, then the needle should swing all the way into the blue section of the meter readout.
The lens meter has a High and Low sensitivity setting. This is adjusted by turning the bezel around the light meter port.
Use the High (marked "H") setting for low light.
CdS cells may take some time to stabilize in low light settings, so give it time if it is very dark. Remember this is technology from the 1960s. Later Gallium Arsenide or Silicon blue cells have instantaneous response.
The Low sensitivity setting ("L") puts an aperture plate physically in front of the cell to reduce its sensitivity. Use this setting for typical daylight exposure measurement. The Leicameters also use the same trick to adjust their sensitivity ratios. CdS cells are also subject to temporary blindness after exposure to excessive light. So, if you aim the camera at the sun, not only could you burn the shutter (stainless steel, but it has been reported that a hole could burn through), but you may blind the meter. That is, it could take several minutes before it can respond accurately to light level changes.
Film is not complicated, but it does require understanding of what is going on. These old cameras expect you to do the thinking. There is no "AUTO" function.
Stephen Gandy has a great writeup on the Canon 7 series of cameras here: https://cameraquest.com/canon7sz.htm