In response to must be based in country (The chicken & egg application problem)

In China, the solution is to NEVER enter the country for employment as a foreign teacher without a Z visa in your passport.

Schools, or individuals, that advise you to do otherwise are unable or unwilling to provide you with the necessary documentation.

The current regulatory environment in many provinces and districts in China makes it difficult for 'schools' to attract enough qualified teachers that will pass PSB review. The solution for many 'schools' is to advise you to enter the country without a Z visa in an effort to coerce you to work illegally at YOUR risk.

Other schools will make do with fewer foreign teachers and/or work harder to attract FTs already employed in China.

This is not a "chicken & egg" problem. In fact, it is a method for the applicant to identify legal and ethical employers.