This question occurred to me when watching my little brother play Battlefield 5. Were the native inhabitants taken prisoner, relocated, or something else? Was life the same after the war for the inhabitants of the islands?
I'll provide what knowledge I can as I am a native of Saipan.
The natives of the Northern Mariana Islands fall generally under Chamorro, Carolinian, and Palauan. Stories from my grandfather during the Japanese occupation many natives were forced to learn Japanese and work to grow sugar canes. During the battle for Saipan many civilians hid in bunkers and caves to avoid artillery and gunfire, unfortunately American troops would at times would use grenades to clear these areas leading to casualties. Towards the end natives were rounded up and placed in a holding camp until roughly 2 years afterwards that eventually called Liberation Day, which also ironically fell on July 4.
After the war the the islands were placed in a trusteeship until roughly the 1970s at which point the Commonwealth of the Northern Mariana Islands was formed. Guam had rejected reunification after WWII due to Chamorros from the other islands assisting the Japanese administration and harsh treatments from the occupiers.
Hope this info helps!