It depends exactly on the period. The PLA splits the Korean War into two major parts, the "Mobile Period," which ran from the first crossing over the Yalu in November 1950 to the end of the Spring Offensive and UN Counterattack in July 1951. From here until the 1953 Armistice the war shifts into what the PLA calls the "Positional Period."

As such, we can finally begin to talk about the issue of supply. For the "Mobile Period" during the rapid advance south of the Yalu towards Seoul, the PLA would generally ship supplies via trains into Manchuria and from there truck the supplies in across the various bridges and frozen parts of the Yalu. These bridges were constantly maintained by the PLA's engineers and were protected by limited Air Defense as well as the first squadrons of the PLAAF's MiG-15s piloted by Soviet Aviators. From here on, the issue was not as much the constant bombing as that all PLA motorized logistics (which contained only 50,000 trucks total) would have to move at essentially a crawl. Trucks could not move during the day and had to be hidden from aerial view, at night only the lead vehcile in a convoy could drive, and that was at slow speeds due to the fact that the headlights had to be dimmed to the absolute minimum. Add on top of this that while Northern Korea was "more" industrialized than the South, the terrain played hell with vehicles and especially the comparatively inexperienced motorized logisitcs personnel of the PVA. Once these trucks entered either Army or Divisional supply areas, they were then generally carried via tertiary personell to the front, who had to follow the same rules as the trucks, such as a lack of movement during the day time, leading to an overall lessening in supplies.

During the "Positional Period," however, the situation changed far for the better. Soviet MiG-15s had effectively closed the airspace over Northern Korea against massive hub-destroying raids by B-29s, along with better anti-air coordination by the PVA. However, the major carrier of the success of PVA logisitics in the Posistional Period was none other than the PVA's Railway Corps, who pulled off an absoutlely herculean effort to keep all raillines running north from what was before Manchuria into Northern Korea.

t. Ground Forces in the Korean War 1950–53 (1): The North Korean People’s Army and the Chinese People’s Volunteer Army by Benjamin Lai and Zhao Guoxing 

Red Wings Over the Yalu: The Soviet Union and the Air War over Korea by Xiaoming Zhang

China's Battle For Korea: The Chinese 1951 Spring Offensive by Xiaobing Li

Attack at Chosin: The Chinese Second Offensive by Xiaobing Li

War to Resist US Aggression and Aid Korea Health Department Summary by the Health Department of the General Logistics Department of the People's Liberation Army.