Why can China go to space？

At 13:34 on September 17, the Shenzhou 12 spacecraft successfully landed at the Dongfeng landing site. With the report of "The three astronauts are in good condition, the Shenzhou 12 manned space mission has been a complete success!" The sound of the report came, and the Shenzhou series manned spacecraft successfully completed the mission once again.

The three astronauts performed missions in space for three months, which is the first time in history. What did they do in the space station and the spacecraft, and how did the astronauts and the reentry module return to Earth?