Apple now allows users to transfer photos stored in the iCloud photo library to Google Photos. The right to data freedom is thus supported by Apple (or, to put it negatively, fulfilled in advance) and allows users to switch providers while keeping their own data.
Thus, it may well be that other cloud photo services will be supported in the future. When using this feature, make sure to omit anything Apple-specific, such as moving live photos. You will only get the standard photos transferred. Metadata can also be omitted.
If you want to use this option, you can log in here to the privacy area of your iCloud at Apple and then go to Transfer to trigger the corresponding function.
Apple states that (logically) there must be a corresponding amount of storage available on the new service, and that it may take a few days for the process to complete. In the meantime, the copying process is carried out and at the same time it is also checked whether the request really came from an authorized user. By the way, the original photos at Apple remain until you explicitly delete them.