>>35278
As far as Thailand is concerned, they had ample contact with Chinese traders from the Guangdong region since the 13th century.
Thailand's neighbor Burma eventually started making trouble, which greatly unnerved the Chinese Qianlong Emperor, causing the Sino-Burmese war of 1766 which turned into a complete disaster for the Chinese. Around the same time, Siam's capital city of Ayutthaya fell to Burmese forces.
For what is was worth, however, the Chinese attack(s) on the Burmese distracted the Burmese forces in Siam, so that general Taksin (himself the son of a Chinese immigrant and a Thai mother) could gather his own troops and drove the Burmese out of Ayutthaya mere seven months after it was captured. Taksin shortly afterwards became king of Siam, and actively encouraged trade with an immigration from China.
Later on, all the shit going on with the Qing dynasty and the massive population increase in China caused further migration waves of Chinese people towards Thailand - at first especially Chinese males that intermarried with Thai women, but later on entire families would move over, to the point that Chinese people made out almost 12% of Thailand's total population by 1912.
Don't know much about Cambodian Chinese, but from what I could find, they basically fill the "happy merchant" niche over there (ironically many Khmer Rogue who would ruin the Sino-Cambodian business where of mixed Sino-Cambodian ancestry themselves). They only pose a relatively small part of the population, but are highly influential in business and politics.