explain why it is summer in the Southern hemisphere and winter in the northern hemisphere in December

Earth's tilted axis causes the seasons. Throughout the year, different parts of Earth receive the Sun's most direct rays. So, when the North Pole tilts toward the Sun, it's summer in the Northern Hemisphere. And when the South Pole tilts toward the Sun, it's winter in the Northern Hemisphere.

because the center-south is called the heart of the country

i think the answer to your question is: settlement of the interior and economic growth. ( ^_^)／