The service is pretty straightforward: photos and videos are sent directly to iCloud, so there’s no need to download anything. You can send your whole library at once or specify which albums you want transferred. Google has a list on its support page of the 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 delete them manually.
Google is the latest company to offer a tool like this; as part of the Data Transfer Project, Meta released a tool in 2019 that lets you move your Facebook photos to other platforms. Being able to take your photos with you makes it easier to preserve memories and switch between phone ecosystems, should you want to do that.
Big tech companies working together? Unheard of! Letting you take your data with you between platforms is, you know, the right thing to do — and incidentally, it’s the kind of thing regulators love to see.