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How can you transfer photos/videos to icloud so that the storage in' General' is not exceeded?
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Sorry, but you can't store photos or videos in the iCloud Photo App and delete them afterwards from your phone.
All the changes you make on your iPhone inside the photo app are reflected to iCloud and all other devices, that are connected to your iCloud account. Deleting photos from your phone will erase the photos in iCloud/Photos as well.
In this case, transferring the photos and videos to your computer into a separate folder or external HD would be the better solution:
Transfer photos and videos from your iPhone, iPad, or iPod touch to your Mac or PC - Apple Support