Is it possible to retrieve other user's public photos using Google Photos Library API? I don't see any such thing in the Documentation. All I can retrieve is my own uploaded photos.
Google Photos does not support public photos. You can share an album via a URL or directly with other users of Google Photos.
In the Google Photos Library API, you can retrieve a list of the shared albums the user has access to via a call to sharedAlbums.list and share or join an album via albums.share and sharedAlbums.join and. When an album is shared, you can also retrieve a shareableUrl, which is a link to the album in Google Photos. Anyone with this link can access the album and its contents.
(If you want to implement sharing in your application, you should look at the share media developer guide to get started.)