I noticed the OnePlus file manager isnt on the play store so I wanted to keep a backup in case I wanted it again. I decided to ADB pull and noticed it's in the system partition. I guess it gets updated via OTAs and installed after a factory reset.
You don't need root or ADB, just a file explorer capable of showing the root of the file system. Go to /system/reserve and select the OnePlus app you want back by running the apk install from there.