Comment by lostlogin

When you first turn on iCloud Photos, it can take a while to upload your photos to iCloud. You can continue to use Photos while your photos are being uploaded. To have iCloud Photos sync photos to all your devices, sign in to the same Apple Account to turn on iCloud Photos on all your devices.

https://support.apple.com/en-gb/guide/photos/phtf5e48489c/ma...