Hinduism: earliest world religion to influence the region, spread by Indian traders, many have blended Hindu gods with their own spirits and beliefs, accepted Hindu belief of reincarnation, rejected ideas like rigid caste system, Hinduism later declined, found in Bali and parts of Malaysia.
Buddhism: far greater impact than Hinduism, missionaries carried religion eastward, Theravada Buddhism took root in Myanmar, Thailand, Laos, and Cambodia. Local people built monasteries, and Buddhist traditions shaped their culture. Boys entered a monastery at an early age. Learned to read/write/become monks.
Chinese Buddhists lead to the spread of Mahayana Buddhism's presence in Vietnam. they believe ordinary people and monastics can attain enlightenment. They too believe men and women can achieve nirvana. buddhist influence remains in SE Asia strongly, people often blend with animism Islam and Christianity have also spread predomiantely in those random island places