The cloud back up would happen as the following.

1. Only your local photos would be copied to the cloud.
2. Google photos / Apple Photos, Backs up your local images, They make a copy in the cloud, So as long you have the local photo there may be a copy in both, Google and Apple photo.
3. If you delete de image in your phone you may still have it on the cloud, However if you delete the image from the cloud, They may ask you if you want to delete this file completly.
4. IF you delete the image from Google photos, Apple will have the cloud backup since they are different apps, however in order to delete a picture you have to go to each individually.
5. If you edit a picture in the cloud, Then wont be backed however if you save it as a new file this would have to be backed up again, Which means that could create a new image, Making tedious to delete the original copy which would still being in both. (Edits and need to change it for a new name)

Think of it on this way if you delete any picture from your phone you may need to delete it from both, Google and Apple photo in order to erase it completly.

IF you delete it from any them, Google or Apple photo, One could delete the image, But you may still have one copy. Either Apple or Google but never locally since they erase the picture often from both, The cloud and locally.

[Phone] - > [Google] * [Apple]

Hope it helps.