If you're interested to get MCSD App builder certificate, you have to start from 2 basic certifications:
480: Programming in HTML5 with JavaScript and CSS3 or 483: Programming in C#.
Here you can get full file with ms certification roadmap - MCP Cert Paths_02-27-20. I've chase the first one because I'm more interested in frontend development.
As a developer, I needed to trigger a React modal from a SharePoint Ribbon button, bridging classic JS and modern React. This post explains the challenge, the solution using the global window object, and how I made React state accessible from outside the bundle.