I made an 18 minute video which is too big to move off my iPhone - now what do I do

Use AirDrop to transfer the video to your Mac. There is no file size limitation. You will need to activate AirDrop on both devices and the devices must be no greater than 30 feet apart.

-- Rich

Use iMazing for exporting it to your computer, trial is enough for just 1 video. Then use Dropbox/Google Drive/etc. to put it in the other device if needed.