React build command? rebuild after changes?

I am doing it for the first time so, there is some confusion, I am using create-react-app to build react site in vs code.

In the terminal, it says to create optimized production build use -npm run build
command which is creating a build folder. but after the creation of build folder if I make changes in my code outside of the build folder will it be reflected in the build folder.
or do I need to rebuild the app again and again after every code changes?

I believe you would need to run the build command every time you make changes to your code. Are you wanting to deploy the code, or are you trying to simply view your changes?

After building something i will deploy it

If you have the ability to, you can setup a Continuous Integration Continuous Deployment Pipeline (CI/CD) and specify the npm run build command as a task in part of the process. I just recently did this for Azure, here’s the follow along I watched
Were are you deploying to?

after I finish my first react project, plan it to push it to Github and host it on netlify through Github repo.

Here is the documentation from Netlify on how to implement CI/CD pipelines for their platform. If you aren’t familiar with CI/CD, once you have it all setup (the doc walks you through that) you would then make changes in your code, commit them, and then your pipeline would take care of the rest of the build and deployment process.