PHPUnit how to use in pet projects?

Question

Unit testing

It seems that they are required in all vacancies, but I don’t know how to cook them.
As far as I understand, Unit tests cannot test those methods that climb into some other methods of another class, and those, in turn, either climb into the base or into some other methods. In this case, integration tests are needed.
But if we take a simple CRUD application with the simplest validation of form fields, let's say a simple task manager with registration and authorization, then what is there to test? Validation methods by substituting different combinations of data?
I don’t understand from which side to approach the tests, well, there is a UserController, whose methods create a User model object, then work with the methods of the User model class, receive data, write, validate, etc.
Please give an example of what can (and should) be tested even on small projects?