SG-Beijing Capital Airport-DAT & DAT-PEK-SG

Hi, I will fly in PEK T3 in the evening and will need to catch a domestic flight Air China to Datong
the next early morning.

1) Which terminal is Air China?
2)Is the international airport is separate from the domestic airport that needs to transfer from terminal to another terminal?
3)We need a layover in the airport for about 4-5hrs and we do not want to check in the airport hotel which is very expensive. Any suggestion for an elderly which need some rest? I found out there is an hourly sleep services but how much is per hour cost? Can it accommodate 4 person just need a place for layover?

Thanks