How to start learning Unit Testing?

Question

ASP.NET

Purely for myself, I want to understand how to work and where to start testing the application, I'm interested in a web application.
For example, there is a small crud application that is divided into controllers, services and repositories.
How and what should be tested, is it necessary to test a repository, or a service, or a controller in general?
I saw that there are many of them, such as Moq, XUnit and so on.

Answer 1

How to start learning Unit Testing?

With tests covering simple modules (which do not depend on other modules), which do not have side effects (do not go to disk or network, do not ask the user anything).
A module can be a method or a class.
You can also read the book

I saw that there are many of them, such as Moq, XUnit and so on.

These are tools for solving different problems.
Moq - for creating mock objects
XUnit - directly a framework for describing and running tests.