If a person commit a crime in mainland China & go to HK or Taiwan, police or relevant agencies from mainland China can't arrest the person in HK or Taiwan.
In Feb 2018, a woman was murdered by her boyfriend in Taiwan. Both of them are HK people. The man soon went back to HK. Local procuratorate from Taiwan couldn't arrest him or asked help from HK government considering no judicial cooperation. Then HK's Security Bureau suggested extradition, which is 2019 Hong Kong extradition bill . Taiwan didn't accept it because their government don't agree with One-China policy, but of course Beijing supported it.
HK people were against it cuz they believed it would hurt human rights of them. They worried that people who have different political stand in HK could be taken by Chinese government in the future. They decided to organized parade.
Every year, people in HK honour the memory of dead people in 1989 in the day of June 4th. But this time, it's Thirtieth Anniversary, so HK people have higher emotions. Around this point, any key words about Tiananmen Square in 1989 couldn't be post in Chinese social media. Anything posted by overseas users was carefully censored by government for 10-30 mins. A blogger innocently posted a screenshot of a film's score with 6.4/10, then the account got ban for 30 days(government thought 6.4/10 means June 4th of 1989).
HK protests started to parade on June 9th. Since that moment, the parade never stopped. Even clash happened. It's unclear how many people in there. Different numbers from 200,000 to 2 million. On June 15th, Chief Executive of Hong Kong said extradition bill suspend. HK government later used many different words like "Has suspended" "Immediately stopped" "Put a stop", "Will lapse" "Will expire", "Will die" "Is dead". But Legislative Council of Hong Kong said only the word dismissed is available in HK's laws. HK protests still didn't buy it though the extradition bill probably wouldn't happen for a while(maybe decades).
HK protests actually already achieved what they wanted and won it after that. But the parade has been out of control. It suddenly became a movement that everybody asked anything they want, but no one could tell what exactly it is. Only thing that's certain is violence has been increasing everyday. At this point, western presses keep saying HK police use violence against HK protests. Chinese presses keep posting images or videos that people from mainland China in HK were subjected to violence by HK protests just because they are Chinese. HK protests came to HK Airport & continued to protest against China, causing airplanes can't land in airport.
Just in this week, a Chinese journalist was beaten by HK protests all around him in airport because of Chinese identity. Before they started, the journalist said: I support to HK police, now you can hit me. This line soon has been widely reposted in Chinese social media by people including celebrities. The journalist was rescued by HK police later.