Thank You for question.
First, Vox was a single-playlist player. It tried hard to stay away from being a "Library". Users appreciated this, but some of them were, like, "why only one playlist? I want many playlists, like in xxxx-player on windows!". So the Playlist was replaced by Collections.
But then other users stepped in, "why so many Collections when we loved Vox for simplicity, bring back one-playlist mode!". So we added back the "Playlist", which is , in turn, a Default Collection.
So, to answer Your first question, Collections are playlists.
Because Collections are not part of library, they are not sorted per artist or album - they are controlled by user only - what You put there, stays there, be it artist or album or arbitrary list of tracks. It's ok for users who like to manage everything on a file-folder level - they want to drop files from Finder onto Vox and manage their playlists manually.
If You have any questions, please do not hesitate to ask!