There was an interesting 'Reply All' episode about the town that this impacted - https://gimletmedia.com/shows/reply-all/wbhjwd
They were promised after decolonization, Hong Kong would get elections for 50 years and then China would fully take over.
China was also allowed to implement laws protecting national security.
What the British didn't foresee is China being able to gerrymander and also influence which politicians could run in the name of national security.
My primary take on it is that the local government was overwhelmed and out of their depth and their naivete was exploited with promises of jobs and funding. At one point they declared people's homes uninhabitable to force them out. It's a pretty sad story on the local level.