How could the People's Liberation Army mobilize so many troops for Korea soon after the Second World War?
Short answerIt was already a mass-mobilization army and had not truly demobilized after the Chinese Civil War.
NotesThe answer starts from the PLA's origins as a mass-mobilization army. It contrasts that organization with earlier Chinese difficulties over conscription and control. The crucial point is that the force had not fully demobilized before the Korean War. Korea therefore drew on an army already organized at very large scale, rather than requiring a fresh mobilization from peacetime. The response also describes that large mobilized footing as persisting long after the war.