Deployments during WWII were different from today in that you didn't always deploy from the United States, fight, and immediately come back stateside. There were enormous tasks at hand; pushing the Japanese empire out South East Asia and the Pacific, and Hitler and the Germans, East from Western Europe. Due to the nature and scale of WWII, most enlistees/draftees were signed up for "the remainder of the war + 6 months" and/or were eligible to go home after a # of campaigns, combat flights, or battles.

Deploying depended on how early you were assigned to a unit. You could be part of the initial muster and training stateside to go overseas. (Think 101st Airborne Division, Easy Company training in the South to go to Europe.) If you signed up later in the war, you could be sent over post-Basic and specialty training to a unit as a combat replacement. In a number of excerpts from accounts of WWII, we hear of new replacements being brought in in the middle of a campaign, or during the staging for a campaign to replace casualties. This goes back to your original question. A majority of the units that had served in an engagement, were brought back to a staging area for R&R, refitting, and re-training to take part in another campaign.

This is similar to the 1st Marine Division after the Battle of Guadalcanal being sent to Australia to rest, recuperate, and train to go back out to Cape Gloucester and fight the Japanese. After the Battle of Cape Gloucester they staged at Pavuvu, again to refit and continue the fight at the Battle of Peleliu.

I assume that the process was similar in the European theater but my perspective comes as a former Marine that has studied Marine Corps history more in-depth.