In this article we explain how to make Spotify recognize and find your music from external sources.
To clarify what you're doing here, you're not actually uploading music in the sense that you're sending it to Spotify's servers. Instead, you're adding your local music to directories on a desktop computer to Spotify, and it will then include that content when it serves your collection.
Once Spotify monitors these folders, new songs you place in these folders will automatically be made available in the app.
Click on your user menu in the top right corner of the app window.