How to debug any JavaScript/React based application in browser step-by-step

Many beginners as well as experienced guys still find it difficult to understand how to debug any JavaScript or React based or any web application.
People struggle to find the correct script file being used by looking at the UI or in other words which file to debug from among many files present in the application?
In this blog you will learn all these: How to debug in browser, How to start debugging, Using actual application, Where to start debugging, How to find here to start debugging, Get Source code , Video demo also available