How to create custom Widget in Mendix using web components (W3C)
In Mendix 8 (Mendix Studio Pro), custom widgets are made using React & there is nothing stopping you from using web components inside React widgets:
I would develop your widget using your preferred web component development tools (e.g. litelement, stenciljs or nothing at all ) then just use the react widget as a thin wrapper around it.
You will probably need to pass props down to your web component and maybe map events from the web component to the react widget. There are loads of articles about how to do that, heres one: https://coryrylan.com/blog/using-web-components-in-react
Why is jour profile not public?
You could get started with Pluggable widget using React https://docs.mendix.com/howto/extensibility/pluggable-widgets
Looking at the React does, it should be possible to work int W3C components too.
If you got something working, I love to see the results.
There's also a learning path but I do not know whether it uses W3C tbh: https://gettingstarted.mendixcloud.com/link/path/45
I would say start here: https://docs.mendix.com/howto7/widget-development/