Depending on the software used, some of the EXIF data can be changed by the user to reflect EXIF data that pertains to one of the images used to make the composite. As a result, the most important EXIF data that a photographer wants to be included for whatever reason can reside within the final composite.
Another possibility is that when I combine 10% of one image file with 90% of another image file, I might do so in a manner that ensures that the EXIF data that is embedded in the composite relates to the capture that pertains to the image that makes up 90% of the composite. The result is that most of the original EXIF data applies to the final composite.
More important, when I make a composite image, I add the information to the XMP data to explain everything I might want to be reminded of in the future of how I made it. I also keep the original image files that were used to make the composite, making it possible for me to review the original EXIF data embedded by the camera in each image file. Having said that, I've never needed so far to review that unaltered EXIF data in each of the originals used to make the composite.