How does extending base eslint config mean?

[c10b10]

I'm trying to extend the base CRA eslint config, and reading the guide has confused me a little:
https://create-react-app.dev/docs/setting-up-your-editor/#experimental-extending-the-eslint-config

Does using EXTEND_ESLINT is sufficient for extending the default CRA config?
Or do I need to create or extend a shared-config like in the example? The shared-config part is confusing, as purely referencing it will crash eslint since it doesn't know what that is.