The 24 hour visa exemption allows for transit through a Chinese city for a maximum of 24 hours (so long as the plane makes only one stop in China, and the departure and arrival countries are different, such as Canada -> China -> Australia for example).
These exemptions can be applied for in almost all Chinese airports by people of almost all nationalities, even if there are exceptions regarding smaller airports and certain nationalities (especially Asian countries considered “high risk” such as Syria, Iraq, etcetera).