Do the secondary subsidiaries of central enterprises belong to central enterprises?

Belong to. Subsidiaries are all holding companies of the parent company, and the shares of central enterprises can also be called subsidiaries, so the secondary subsidiaries of central enterprises belong to central enterprises. Central enterprises, referred to as "centrally managed enterprises", refer to the central people's government (the State Council) or the state-owned assets supervision and administration institution entrusted to exercise the responsibilities of investors.