once I have created my app, how do I share it with someone else so they can run it locally
npm run build?
how would they run it locally from the build folder?
They can just open the HTML file from the build folder in their browser. If your app needs a web server to run, you can configure one easily or just upload the contents of your build folder to a web server. One thing you should do is double-check to contents of the HTML file to ensure the paths to your CSS, JS, and other files are referenced correctly.
Unless the person you are sending it to will be making changes to the app itself, all of the NPM commands won’t be interesting to them 