Hi,
This a big questions that seems like it might be related to
changes in the Sun but is really about the way the Earth is tilted
towards or away from the Sun at different times of year. The Sun itself
is the same all year round, but Earth is tipped so that in, for
instance, January the northern part of the Earth is pointed a bit away from
the Sun, which makes days shorter and colder, while the southern part
of the Earth is tipped a bit towards the sun, making days there longer and
hotter. The reverse happens in July. Thus winter in the North America
is summer in Australia and vice versa.

It helps to look at pictures. See this web site:

https://spaceplace.nasa.gov/seasons/en/

Terry