Google Drive and Photos divorce: Who gets what?
Google Drive and Google Photos will soon de-sync photos and videos hosted between the two. Google suggested that "automatic sync" between the two services was just "confusing" for users, so they've decided to "simplify" the experience. Accidental deletions and appearances of counting data twice will soon be out the door – sort of.
Separation Anxiety
It's going to be carnage! In July of 2019, Google will sort-of separate Google Photos and Google Drive. Right now they share hosting of the media you upload to Google Photos. You can access everything you upload to Google Photos from either Google Photos or Google Drive. Soon, things will change.
For the photos and video you have already uploaded before July, only one thing will change. If you delete a photo or video from one, it will not be deleted from the other. Before July, if you delete a video or photo in Photos or Drive, it'll be deleted in the other service too.
App Functionality
Starting in July, your already-uploaded photos and videos (from before July) will remain as they are, but new photos and video will not appear in your Drive when uploaded to Google Photos via the Google Photos apps.
If you upload media to Google Drive on its own, it will not automatically sync with Google Photos. If you use "Backup and Sync" on Windows or macOS to upload to both services, in Original Quality, only one iteration will count towards your total storage quota. If you upload in High Quality, your quota will be unaffected.
If you do not use "Backup and Sync" to upload media to both services at once, each piece of media will count toward your storage quota individually. That counts even if you use "Upload from Drive" to copy one to the other.
Upload from Drive
If you upload a photo or video to Google Drive, and you'd like this media to also be in Google Photos, there's a feature now called "Upload from Drive." This feature allows you to select and copy photos and videos from Drive to Photos without leaving the Photos website. This feature will exist first (and maybe exclusively) in the Google Photos website interface.
Google created a Q&A document to clarify any questions that may remain in your mind at this point. Don't get it twisted!