What is single variety planting?

For example, if China only grows wheat, it will import rice, soybeans, corn and other crops from other countries, and China will export wheat. Is it related to the global food supply?

Planting a single variety is easy to multiple cropping, and the effect on soil fertility recovery is not good.

Single variety planting makes many varieties unable to be self-sufficient, leading to world grain logistics, increasing transportation links and destroying the ecological environment. There are too many negative factors.