Hi Ankesh Singh,
I advise you to follow this learning path that is perhaps the best way to help you.
https://academy.mendix.com/link/paths/45/Building-Custom-Widgets-(Micro-Learning)
BR,
MM
If by predefined widgets you mean their page-elements like drop-down, text-box, etc: no. That is all proprietary Mendix stuff, no open source, not extendable.