How to explain the boundary between yin and yang?

The boundary between yin and yang is just a general term for opposing environments, just like light and dark, cold and warm, which can be summarized by yin and yang. If the world thinks it is the Yang world, then the place where the ghost is located is the Yin world. Lonely yin is not born, and the sun alone is not long, so sometimes you have me and I have you. Just like inside and outside, you can take the room as an example. The living room is outside and the bedroom is inside. For a family, the living room is inside and the door is sometimes outside.