1. How do I see all the songs in my library? Where is the aggregate list Playlists + Artists + Album. This used to be a feature available on Android and iOS a couple of years ago! Why has this feature been removed? Bring it back!
2. Same problem while browsing an artist. It only shows “popular songs”. Where are ALL the songs from this artist that Spotify carries?
1.) I'm afraid that the "Songs" category under "Your Library" went away many updates ago. If I had to guess, it was probably because it wasn't utilized much by the majority of users. If it's any compensation, you can see all the songs you have liked in one place. They are all put in a playlist called simply "Liked Songs".
2.) I'm afraid this also isn't an option anymore either. Once again though, there is something close to finding all the songs. If you go to an artist's page, scroll past "Popular" and "Popular releases" you will see a small button labeled "See Discography". There you will find all there Albums and singles.