Would the code that goes in to the Custom Javascript area be an embed from a third-party app or can it be an app that I build myself (React, CSS, HTML)?

I don't see how I can add an app of my own with just javascript code. I would need to enter HTML.

Would it be embed from a third party app or can it be an app I build myself.

Either will work. If you have a app you build yourself you should host it. Then you can reference it or any other third party app using <script src=‘’></script> example in the help article.

I would need to enter html.

It is necessary to see an example to understand your concern, as this is not the case. The typical practice is that a embedded script will inject elements into the DOM. There is not a need to enter HTML into this area, or, you can do so using javascript, for example appendElement etc. Referencing the script should be sufficient, and all DOM manipulations can happen in the script, as evidenced by any chatbot and the example linked in the original article. Your script should manipulate the DOM. Try building it to demonstrate your concern, as you should be fine.