Does transfer via HKG count for onward travel to SG?

For the 144-hour visa, is this anytime up to 144 hours? So we can depart any time within this period?

Based on the above examples; the following would be eligible?
UK > Shanghai (stay for up to 144 hours) > Singapore (via Hong Kong) > UK
Not sure if HK is seen as mainland China or not? (My understanding is that it's not).