I especially enjoy finding a series of songs which are connected in series. For example, Edwin Birdsong released Cola Bottle Baby in 1979. This is then sampled by Daft Punk in 2001 for Harder, Better, Faster, Stronger; which in turn is used as a core component of Kanye West’s 2007 song Stronger.
It is not uncommon for a song to be used in many different songs, creating branched connections with multiple pathways. The web is very interwoven and chaotic. A great primer for samples and a few famous examples (e.g. La-Di-Da-Di by Slick Rick and Doug E. Fresh) is this Ted Talk from Mark Ronson.
These connections and histories are not secrets (for the most part), and there are websites dedicated to organizing all the connections between songs. I just enjoy collecting as I find them.
Just as interesting - though in a different way - is when I think I have found a sample or interpolation, but it is just two songs which sounded similar. I also keep track of these. Perhaps you can hear it too.