To my understanding, in order for some landmass to function as a part of a country, three items are important:
- a common constitutional framework
- collaborative revenue management
- military bases
As a member of a federation, a landmass can abide by any one or more of these factors.
A common constitution can take care of judiciary, parliamentary representation, and so on. The collection of taxes benefits the federation and federal subjects. Military bases benefit the federation to project power.
Now, my question is, how are China and Hong Kong sharing these factors? Or, are they really sharing anything? If not, how is Hong Kong considered as a part of China?