Short answers, pulled from the story.
Mahayana Buddhism arrived in the Kingdom of Funan during the 5th century alongside Hindu merchants. Chinese traveler Yijing observed this dual religious presence toward the close of the seventh century.
King Jayavarman VII reigned from 1181 to 1215 and worked tirelessly to establish Buddhism as the state religion of Angkor. He became a Mahayana Buddhist who regarded himself as a Dharma-king and bodhisattva.
The communist Khmer Rouge tried to completely destroy Buddhism when they took control of Cambodia in 1975. Estimates suggest that 50,000 monks died during the Khmer Rouge regime and nearly every temple and library had been destroyed by the end of their rule.
Estimates of the number of monks prior to the ascension of the Khmer Rouge vary ranging between 65,000 and 80,000. By the early 1980s the number of Cambodian monks worldwide was estimated to be fewer than 3,000.
After the 13th century Theravada Buddhism became the state religion of Cambodia following the return of Prince Tamalinda from Sri Lanka. King Jayavarman VII sent his son Tamalinda to Sri Lanka to be ordained as a Buddhist monk and study Theravada Buddhism according to Pali scriptural traditions.