East Asia has no one dominant religion. Buddhism is certainly
the most common religion, but since Buddhism lacks a strong
orthopraxis and a strong orthodoxos, it can be mixed with local
beliefs, such as Dao, Confucianism, Shinto, and Cao Dai.
Additionally, many East Asians follow Christianity and Islam as
informed by Buddhism.
It is also worth noting that there are different general strains
of Buddhism, like Theravada Buddhism, Mahayana Buddhism, and Zen
Buddhism. It is not a monolithic religion any more than
Christianity, Islam, or Hinduism.