what are the VISA requirement for Chinese Canadian with USA GREEN CARD working in China and HongKong

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I am Alex. I am Chinese Canadian, born in Hong Kong, and become Canadian Citizens many years ago. I have Canadian passport. I also have a USA GREEN CARD now, but not a USA citizen yet. I am going to help build a new line of business for an existing business entity in China. They are willing to hire me to be the head of the department. I am finding out what is the best way for me to gain longer term working permit in China so I can stay extended period of time.

Regarding the requirement "First, the organisation must be accredited to employ foreigners"..... how does the business entity in China obtain this accreditation. Who in the business entity need to authorize me ? The owner of the business or the CEO should sign the application for accreditation ? Or the HR manager of that organization ?

Where to get this application ? Which branch of government should be contacted to get this application of accreditation ?

What if we open a branch in Hong Kong and I also will be working in both mainland China and Hong Kong - do I need different application and visa to also work in Hong Kong at the same time ?

Thanks.
Alex at USA