All movies recorded on an iPhone with the Camera app are MOV files, be it "More Compatible" (H264) mode, "High Efficiency" (H265) mode, or "Apple ProRes" mode.
How are you transferring our iPhone Apple ProRes video recordings to your Windows computer? Some methods will transcode large video files (like ProRes) to smaller files to make transfering video more low-bandwidth friendly. Of course, when it's footage that we plan to use in our edit we want to avoid this.
Here's Apple's official list of options for transferring files between our iPhone and our computer:
To avoid unintentional transcoding and to better work with large video files, I'd go with Transfer files between your iPhone and computer with an external storage device.