As a student I am looking for the mostly used GUI framework in the industry for standalone desktop apps when all the frameworks in all the languages are considered together. Basically I want to know the one with the highest market share.
My requirement is just to start learning it and I don't want to target specific application types.
What platform are you interested in? For cross-platform apps the industry seems to be moving toward Electron apps like Slack or Visual Studio Code is built in - it's a framework for embedding Chromium into a desktop app.
For Windows only, there is Windows Forms or Windows Presentation Foundation.
On Linux, Gtk is popular.
In Python, TkInter is included with the language.
Basically I want to know the one with the highest market share." - then Google is your friend. Please read How to Ask and understand why this question is off-topic and will be closed unless you reword it.