Please refer to the following answer to get a basic extension app running.
https://forum.developers.symphony.com/question/1351124750339936256/test-extension-app-in-sandbox#post-1351127220696911872

Once you get the sample running, you can modify app.html / js to include anything you want, including UI frameworks etc. Everything in app.html / js is entirely your implementation so you can include any framework you wish. Obviously, we cannot possibly provide support for Angular itself and any other frameworks that we do not own.

For Angular, we need few dependencies to be installed which I installed in the same extension application folder. How do I connect app.html to my angular code?
When I use the angular code now, it isn't able to compile that.

Correct. I get the point that the entry point needs to be replaced. My question is exactly that. How do I make app html/js of my angular project the entry point?