Are we in a content script or background script?

[bitttttten]

I am writing an extension that talks to all the tabs, and I am looking into the Chrome docs to understand how it all works. I can't figure this part out though, in my React App inside a hook I'll be setting up some listeners and so on, and I am wondering if I should see the React App as being part of the content script or background script.

For more context: I am making an extension that you can communicate with a React App. They will send messages to each other.

Thanks!