Google Photos and Mac Photos

These three services are all completely independent, and their photo libraries aren’t connected to each other. Because of that, you don’t necessarily have to do anything different. If you want to use Lightroom Classic to centralize how you manage photos, and you don’t want to use Apple Photos or Google Photos any more, you could export all files from Apple Photos and Google Photos to one or more folders on your Mac, and then import those folders into Lightroom Classic.

You can also set the Lightroom phone/tablet app to automatically import photos taken with the iOS camera. The Lightroom app would then use the cloud to sync those photos to Lightroom Classic, which will download a local copy of the photo to your Mac.

What gets complicated is if you want to use Lightroom Classic and its Lightroom Photos cloud service, while continuing to use Apple Photos and Google Photos. (For example, if you want to continue using the cloud sharing features of those services.) Because the three services don’t connect to each other, you’d have to plan carefully to understand which photos are where, and to avoid unnecessary duplication.

Simple explanations-

Point#1  LrC requires photo files on a local hard-drive.

https://www.lightroomqueen.com/where-store-your-photos/

Point#2 Mac Photos App 'locks' files away in a Library that is not accessible to LrC

MAC PHOTOS Explained

Point#3 With Google photos If the files are only in the Google Cloud LrC cannot access them- there has to be a full local copy for LrC.

(Other users of Google Photos will need to answer here)