Grafana has both core and external plugins. xychart is a “core” plugin (meaning it is part of the grafana core codebase) and as you pointed, its code is inside the grafana repository grafana/public/app/plugins/panel/xychart at main · grafana/grafana · GitHub
Your contribution is very welcomed!
you can see the general contribution guide and the developer guide to setup your environment to work in the Grafana code base
In case you are using Windows, you must use WSL for all the development of Grafana.