[germinalphrase]

Is there any theory/ideas about how a "post-growth" economy would/could function?

[asdfgasd]

There are many, it's a very old, very well thought-out tradition. Try reading Kropotkin, or from a more modern perspective, Richard Wolff.

However, it doesn't align with the current vision of orthodox Western capitalism, so it isn't particularly popular in mainstream media or culture.