Why are Hong Kong people originally from Guangdong Province?

Not all of them, but most of them are from Guangdong.

First of all, before the mainland closed its doors in the 1950s, Hong Kong was free to come and go, and it was free to cross the border. There were often wars on the mainland at that time. Naturally, many people used to run away from war. Guangdong is close to Hong Kong, so naturally there used to be many people in Guangdong.

Then in the 1950s and 1970s, due to the economic downturn in the Mainland and several political activities, many Cantonese people sneaked into the past to work, even though the customs were closed. Especially in the 1970s and 1980s, Hong Kong implemented the policy of reaching the base. As long as the stowaways are not caught by the customs, they are issued with identity cards, which makes more people sneak in to work.