Why is ReactJS not working?

Question

React

I would like to apologize for a possibly stupid question. I ask you not to beat for the incorrect use of terms and if additional information is required, then I will be happy to provide it.
There is such a situation, the spring boot project to which I would like to connect react js so that you can build beautiful interfaces.
There are two files, one is index.js

The second file, main.html


I wanted the phrase "Hello World" to be inserted into the main.html file using id kek.

After accessing the main.html page from the server, I get this response:

Answer 1

Have you completed your project?
npm run build
This will build the project into a *.js file, which you must include manually in your main.html
If I understand the batch correctly, of course.

Answer 2

ReactDOM.render() not just render()

Answer 3

js file must also be included in html, it will not appear there by itself.