https://www.musical-u.com/learn/learning-to-listen-harmonies/
Music is amazing in the way that something which initially sounds simple can actually be extremely complex when you dive into the sound and actively listen.
An example of a very basic song would be a simple tune such as “Hot Crossed Buns” which is just a melody.
You could in theory do many different things to improve that simple melody, but one of the easiest things to do would be to add harmonies.