Before the digital age, royalties were difficult to track, but there were fewer platforms to consume music, so there were far fewer royalty streams to worry about.
With physical sales plummeting, and people shifting from downloading to streaming (like Spotify and Apple Music) and the rise of digital radio (like Pandora and Sirius/XM), there are many more royalties out there, but they can be tracked much easier through sonic recognition and content ID software.
We're not quite there yet, but we're getting closer every day.
For indie artists without a label or a publisher, you have to know what these royalties are and know where and how to get them.
Ari Herstand has done a noble service with this informative list of revenue streams for the new-to-it-all artist and songwriter (and it's a good refresher to those who have been around the block, too). I love pieces like this as they are useful to bookmark and pass on when I encounter an friend or client who wants to learn more about how the sausage is made.
If you'd like to continue your studies, here's an article titled Now You Know Everything About Music Publishing, as well as a second, completely different article also titled Now You Know Everything About Music Publishing, both hosted by Digital Music News. Now that's unnecessarily confusing, but appropriately a metaphor for music rights management itself.