How does the Git Hosted runners work with these images in Scale

[pmatthews05]

Please forgive my lack of knowledge here, I'm more of a software developer than an infrastructure person.

I've used these images in the past as self hosted runners in Azure DevOps. I created the images, and stored them in Azure Compute Gallery. I had a VMSS which used these images, and following the setup with Azure Devops to VMSS I was able to create Self Hosted runners that scaled up and down when required.

However reading through the documents in Microsoft and GitHub, to create Self hosted runners in scale the options seem to be to use App Containers, or Kubernetes. These do not pull (as far as I have understood) VM Image definitions, they expect an image from Docker or Azure Container Registry.

So how do the Git Hosted runners work that they can scale up and down as demand uses this repo? What method is used, and how are these images that get created in this repo get pulled? How can I replicate it in a self hosted environment with the images created from this repo.