I was struggling with UI and IMHO Blender should put a UI template like this in the templates. The current templates for UI are very basic only covers the operator. Let me know please if I am wrong. May be I have missed it:
Just right-click on the UI element that you want to edit and click “Edit Source”
Btw you need to enable developer extras in the preferences.
Yes, I know that. Thanks for the tip though.
Is there any template available for PropertyGroup?
This was a very confusing endeavor for me.