Teaching english in china is highly fragmented. Unlike other industries where one or a few companies dominate, English teaching in China has many big, medium, small, and micro-companies. Big companies are not necessarily the best paid or best places to work.
Simply because of the balance of demand and supply, many people who can teach English well tend to get more work coming to them through referrals.
English is the common language at both official workplaces such as schools, big companies, and training centers, as well as informally at places like coffee shops that feature English teachers.