The China to Hong Kong border is already open for foreigners (first arrival on 15 September) without quarantine. But only HK residents can fly in from China, the rest of us have to do the land border crossing from Shenzhen, Macao or Zhuhai unless you fancy staying in a quarantine hotel for 7/14 days.

Of course going the other way entails 14+7 days of quarantine and monitoring. But word is that this may change starting from mid-December with a strict quota and then expanding further in February.

If you can overlook the quarantine, going from Hong Kong to China is actually surprisingly easy. You don't even need a pre-flight PCR test, just get your ticket to a mainland city and off you go. Or you can cross at one of the two land borders but that requires a prior booking (just a formality as not that many people cross due to the quarantine) and a PCR test before crossing.