Short answers, pulled from the story.
Buddhism in Cambodia has existed since at least the 5th century. Unconfirmed Singhalese sources trace an even earlier mission to the 3rd century BC, during the reign of King Ashoka, when two monks named Sona and Uttara were said to have been sent to mainland Southeast Asia following a council held in 274 BC.
Theravada Buddhism is the official religion of Cambodia, enshrined in the Cambodian constitution. It has been the state religion since the 13th century, with the exception of the Khmer Rouge period (1975-1979). As of 2019, an estimated 97.1 percent of the population are Buddhists.
The Khmer Rouge attempted to completely destroy Buddhism after seizing power in 1975, classifying it as reactionary. Monks were expelled from temples and forced into manual labor; uncooperative monks were executed. Estimates suggest 50,000 monks died during the regime. A monastic community of between 65,000 and 80,000 monks was reduced to fewer than 3,000 survivors worldwide by 1979.
Jayavarman VII (1181-1215) is considered the most significant Khmer Buddhist king. He regarded himself as a bodhisattva-king whose duty was to save the people through service and merit-making. His reign redirected royal resources from elite Brahman temple construction toward libraries, monastic dwellings, and public works, and shifted temple art from Hindu to Buddhist iconography after 1200.
The two orders are the Maha Nikaya and the Dhammayuttika Nikaya. Since 1855 the Maha Nikaya has been by far the larger, claiming the allegiance of a large majority of Cambodian monks. The Thommayut (Dhammayuttika) order was introduced into Cambodian ruling circles from Thailand in 1864 and remained geographically confined largely to the Phnom Penh area.
The transition was driven in large part by Tamalinda, believed to be the son of Jayavarman VII, who studied at the Mahavihara Monastery in Sri Lanka from 1180 to 1190. He returned as a Thera capable of administering ordination into a disciplined Theravada lineage that rejected Mahayana Tantric practices. Wandering missionaries from Mon-Khmer-speaking regions aided the spread, and by the 13th century Theravada had become the state religion.