But i still don't understand how can i create or modify my own countertop with a customizable hole in it.

Best way to customise your kitchen is to construct it from standardised models.


This will take care of your problem.
Kitchen cabinets, sinks, faucets:
https://www.polantis.com/search/objects ... en+cabinet

www.polantis.com
Thousands of free 3D objects. Everything home designers need.