iMovie for Mac doesn't yet support the HDR video from your iPhone 12. It makes it overly bright. You would need to go to your phone settings and turn off HDR for recording future videos. Go into the camera settings and turn off HDR under the Record Video sub menu. Once the HDR setting is off any future recordings should work in iMovie for Mac.
If you cannot redo your current HDR videos iMovie on your iPhone supports HDR. You could use iMovie for iOS. Or (and I'm not sure that this will work) you could try importing your HDR video into iMovie on your iPhone, and then use AirDrop to transfer the iPhone iMovie project, as opposed to the video, to your Mac. It will go into your downloads folder. Double click on it there and it will open an iMovie for Mac project with the video in it. See if iMovie has converted your HDR video so it will play properly. As I said, I'm not sure that it will work, but may be worth a try.
-- Rich