I see this question a lot and as someone who has traveled to and sometimes lived in Hong Kong for the past 20 years, the truth is on a day-to-day level, there isn’t really much change.
I’m sure if you’re an activist there are huge changes. But as a regular person, life feels the same (even my local and expat friends feel this way).
Also one thing to be clear about is Hong Kong never had a democracy. The governor was always appointed by the UK. It was only when the handover was certain, the UK tried to introduce voting. The UK also tried to pressure Portugal to stop giving citizenship to the people of Macau, because the UK didn’t want to do the same for Hongkongers.
Only part of Hong Kong, the New Territories, was leased from China. Hong Kong Island and Kowloon were ceded to the UK in perpetuality.
The UK didn't have to give them back but it also didn't have a reliable fresh water supply for such a large city that didn't depend on mainland China.
PRC is legally fully compliant with her UNCLOS accession obligations.