You can request to transfer a copy of photos and videos you store in iCloud Photos to Google Photos. Transferring photos and videos from iCloud Photos doesn’t remove or alter the content you store with Apple, but sends a copy of your content to the other service.
The transfer process takes between three and seven days. We use this time to verify that the request was made by you, and to make the transfer… More here.
This makes sense because it’s a pain to backup a lot of iOS data.