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.

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?

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.