Transit at Beijing Capital international Airport

Actually, you need to follow the procedure as follows:
Arriva at the Domestic Arrival Level → proceed with international flights transfer procedures at the D-I Transfer Counter → go to the Transfer Hall → check boarding pass → go to Customs, Immigration and Quarantine → go through the Heath & Quarantine Inspection → pass through the Exit Frontier Inspection → accept Security Inspection → accept Customs Inspection → Waiting & Boarding