How to begin creating a custom, private app with Node.js?

[Alocard]

Hello developers,
I have recently ventured into app development. I would like to develop some kind of "invite members" app.
I want this app to be private and not public. So using the Shopify CLI to install an app template is out, as this installs a public app on a development store.
So I create an app in the Shopify admin of a store and install it. I now get an API key and can set the access scopes. So far so good.
The problem is that I don't know what is the next step then. How do I start using Node.js to build the backend of my app? How do I link my code to the app?