Device Storage
I reached the limit on my device and purchase additional storage on my iCloud. How do I transfer my photos from my device to the iCloud?
iPhone 11 Pro, iOS 14
You can sync your photos but you cannot offload them to iCloud. Go to Settings > Photos and enable iCloud photos. You can turn on optimize too which should significantly reduce the size of the library on your device (originals are kept in the cloud and lower-res versions on your device)