You’re probably gonna have to experiment a bit. For baking and more delicate things probably the listed fan temp, but for things like roasted potatoes it doesn’t matter as much.

The photosites are continuously reading the light falling on them. When you take a picture, the system takes discreet readings of those values for the length of your shutter speed. However it can only handle the information from a portion of the sensor at a time. So it reads in sequential stripes. The longer the exposure time the more of a chance that the sensor or the subject has moved before the camera reads all the stripes.