Why does China have a small Indian Ocean?

I watched the "weather forecast" video according to the landlord's prompt, and found that this enlarged thumbnail did not always appear on the screen, but appeared at an appropriate moment according to the needs of the host's explanation.

It must be clear that the South China Sea is also an inalienable part of the territory of China, and a map of China must fully show the territory of China.

On many other maps, especially the administrative map of China, there is such a small map, but it is often placed in the lower right corner of a larger map.