My iPhone 13 Mini iOS 16.5.1 is filling up with storage when I try to transfer videos onto my PC. I'm trying to transfer a 6 GB video and I have 5 GB free. When I try to transfer, the phone says that it's at full storage, but Windows says it has 5 GB free. This is in the span of 5 minutes, so it's not downloading anything.
iPhone 13 mini
If you are transferring photos and videos using iCloud Photos, those photos and video are going to take up space both in iCloud and in device storage.
There is an "Optimize iPhone Storage" setting you can use to give the phone permission to store "space-saving" (reduced-resolution) copies on your phone. I don't know how well it works, but obviously if you are talking about new videos that haven't made it to iCloud yet, the phone has to keep the full-resolution version of them until such time as it believes they have made it to iCloud.
Manage your photo and video storage - Apple Support
iPhone storage filling up after attempting to transfer files to Windows PC
