When Buddhism was spread to China, it was seen as an exotic religion.
The Chinese, therefore, transformed Buddhism to suit Chinese ethical practices, especially the Confucian virtue of filial piety. See The Chinese Transformation of Buddhism by Kenneth KS Chen (1973).
One of the key transformations was the conversion of the male bodhisattva Avalokitesvara to the female Kuan Yin, the Goddess of Mercy.
Avalokitesvara is a central figure in Pure Land Buddhism. He is a male bodhisattva in India (before Buddhism disappeared from India in the 12th century), Tibet, Sri Lanka, and Southeast Asia.
In China, he was depicted as a male in the Tang Dynasty. That changed by the early Sung.
How and why that happened is the subject of a 631-page book by Yu Chun Fang, titled Kuan Yin: The Chinese Transformation of Avalokitesvara (2001).
To my knowledge, this is the only English-language book on this topic. It is touted as a tour de force by a reviewer from Princeton University.