Why do the seasons happen and why are they different in the northern and southern hemisphere?

The seasons are caused by the position of the sun, the position of the Earth, and the tilt of the Earth on its axis.
The region above the equator is known as the Northern Hemisphere.
The region below the equator is known as the Southern Hemisphere.
When a hemisphere tilts towards the sun, it receives more direct sunlight for longer periods of time, causing warmer temperatures and longer days (summer).
When a hemisphere tilts away from the sun, there is less light and lower temperatures (winter).
Since only one hemisphere can tilt towards the sun at a time while the other tilts away, the seasons are inverted in the hemispheres.
While the Northern Hemisphere tilts towards the sun for summer, the Southern Hemisphere tilts away for winter, and vice versa.

Hope this helps :)