In what season, sunlight enters the room the most? What is the reason?

To be precise, when you are in the northern hemisphere, you get more sunshine in winter, because in winter (assuming that one day is the winter solstice), the direct sunlight is at 23.5 degrees south latitude, and all parts of the northern hemisphere are at the lowest sun height in a year. Imagine, for example, the height of the sun in your place, which is 36.5 degrees in winter and 83.5 degrees in summer. In summer, the sun rays enter almost vertically, so the area with the ground is much smaller than that in winter, so it is winter.