The service is quite simple: photos and videos are uploaded directly to iCloud, so there’s no need to download anything. You can upload your entire library at once or specify which albums you want to transfer. Google has a list on your support page of file types you can transfer, which includes common image and video formats. And .webp, for some reason. Initiating a transfer sends a copy of each file to your iCloud account, so the originals will remain in Google Photos unless you manually delete them.
Google is the latest company to offer a tool like this; As part of the Data Transfer Project, Meta launched a tool in 2019 that allows you to move your photos from Facebook to other platforms. Being able to take your photos with you makes it easier to preserve memories and switch between phone ecosystems if you want to do that.
Big tech companies working together? Unheard of! Allowing you to take your data across platforms is, you know, the right thing to do – and incidentally, it’s the kind of thing regulators love to see.