Extensions are just websites.
In the developer console you tell which HTML file to load into each slot.
So you’ll tell the dev console to load config.html for example and then in the config.html file you’ll add whatever HTML you need as required.
So in this example I use the panel/index.html file
Which is test mode I use the base URI of https://barrycarlyon.github.io/twitch_extension_debug/
This “test” extension is hosted on github pages.
So I could change this to config/index.html and then I’d edit config/index.html accordingly