With a computer, you can transfer all of your video files via iTunes.
To do that, please connect your devices to your computer via a lightning charging cable, and start iTunes on the computer. In iTunes, select your device, then select the File Transfer tab, and then select MultiCam. You should see a list of all of the video files stored within the app.
Drag these files to the computer, or drag them back from the computer into the app.
To build the final composition, please drag video from all devices into the computer, then from the computer into the device used to mix. Then, start the app on that main device, go in the Recorded Media list, and open the journal. The final composition should start automatically.
Instead of using a USB charging cable, you can also use iTunes over WiFi. To do that, enable "Sync with this iPhone/iPad over WiFi" in the iTunes device Summary. However, we recommend using USB because doing so is faster and more reliable.