what kind of question should I do so that I can master the concept of oops, function, and class ??
The question in the title and the body have related but have slightly different answers.
Coding skills are about how well you apply the rules of a language to write a program. It is measured by quality measures - is it easy to understand, easy to change and is it fit for purpose? The preferred method to get better at coding is to read and write code. What works, what is understandable? There are a lot of rules that may help you to think about code, SOLID, Clean Architecture, MVC, and I could go on, but IMHO reading and writing code is key.
The concepts of OOP, functions and classes are tools that a programming language offers to reach the goal of writing programs that better fulfill quality requirements. Better understanding these can be achieved by reading about these - tutorials, Stack Overflow, mailing lists - and applying them. Again, reading and writing code are key here.
There is no silver bullet here. Go ahead and write some beautiful programs.