This is according to Dan Harmon (or more precisely my recollection of what Dan Harmon said about it on his short lived podcast Whiting Wongs.) At the beginning of the season the showrunner has a general timeline for the episodes. Writers will take on the job of “breaking out” (writing the basic skeleton of the episode and the progression of the A and B stories) a few episodes each before the whole room adds their input. Typically the writer(s) who broke out an episode are the credited writers for that episode, WGA rules state that only two writers can be credited for the episode for awards eligibility. Sometimes the showrunner is one of the credited writers but because they are also a producer and therefore eligible for other awards that’s not always the case.

Also, the credited writer is typically on-set to make in-the-moment adjustments when needed.