It all makes sense now!

Inside of this example would the sets that are changeable inside of .override like bar for example be the definitions made within the .nix file for a respective package? I’m trying to understand how I know what can be overridden.

So If I am understanding this, Overlays are a way to apply changes to the package via its definition using already existing mentions like overriding and such then propagating those changes outside to an entire set of packages.