How to set up cloud-init

As I read most of the cloud-init documentation and watched some youtube videos about cloud-init I am still confused about how to use this software.

Can someone help me understand?

As I understood so far cloud-init should configure a linux machine (instance) from outside, so that I dont have to login manually and configure e.g. disk setup, network, users, and so on...

Now I have first use case. A physical computer with an empty hard-disk and I want to install ubuntu on it without manual interaction using cloud-init. How can this be archived?

Or another use-case: I run proxmox (kvm) on debian and want to create an new vitrual machine running ubuntu.

Does anyone know how to do that?