How does react work on a server in python?

I am beginning to learn react as a hobby, I am trying with a simple little project but I already have a difficulty that I do not find in other questions/answers:
I created the project via npx create-react-app my_app, in development via localhost:3000 the default page works, then I tried npm run build and the build folder was created

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then I tried with my own test server in python pointing to the index.html page and I expected it to work the same, instead I have a page that doesn’t seem to take the css.
I noted that in the sources it looks for a couple of folders that are not included in the build folder:

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I have no errors but I don’t understand why, do you have any idea why?
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Sorry in advance for the triviality of the question but I’m new to front-end, I know a little html/js/ts/css but I’ve never used a framework and haven’t found much on the web, thank you very much!

Search in web, try to modify index.html in build folder