Please read also How to upload to Google Photos for other upload methods
Backup & Sync (B&S), a Google Drive app, was launched in July 2017 and was intended to replace Google Photos Backup ("Desktop Uploader"). On May 12, 2018, the Desktop Uploader was discontinued and no longer works.
B&S builds a new hierarchy in Google Drive which mirrors the computer folder structure before passing the photos on to Google Photos. We recommend creating a new hierarchy on your computer for new photos. Trying to merge already backed up files will be very difficult. If you don't want your photos uploading to Google Drive, do not use B&S.
See Geeks on Tour video about using Backup & Sync to upload photos and videos from your computer to Google Photos: https://www.youtube.com/watch
Backup and sync Release Notes from Google Workspace (former Google Suite) team
Support for Apple Photos Library (may not be up-to-date)
Please see How to download photos and videos