Apple offers a way to request a transfer of photos and videos stored in iCloud Photos to Google Photos that happens in the cloud, without requiring you to have enough bandwidth and disk storage to do the transfer via export and then separate upload. The excellent support article explains how to go to privacy.apple.com to transfer directly to Google Photos.
The interoperability basis for this transfer is the use of Data Transfer Project servers. The DTP platform is an extensible Open Source platform for server-to-server data transfers specifically for user data as requested by users.
Not only does this solution transfer your photos, it also preserves them in albums.
It does not transfer:
The source code (markdown) for this article can be found on GitHub.