According to what rules did traditional Chinese characters evolve into simplified Chinese characters?

Simplified Chinese characters mainly adopt popular vulgar characters in China since the Song and Yuan Dynasties, and a few of them are newly simplified according to the simplification rules of vulgar characters. The first simplification rule is the evolution of radicals, such as: gold is simplified to the edge of gold, and bone is simplified to the side of moon. Second, only approximate shapes are kept. For example, turtles only keep traditional shapes.