My suggestion is to read some kind of object-oriented programming tutorial about when to create a class for something and when to create abstract classes/interfaces etc.
If you find something very strange or seemingly unneccessary in any of those tutorials, you probably need more practial experience. Try to build systems and see why they become practical/not practical to expand, maintain, understand etc.
When you get more experience you might want to read about design patterns and other software theory.
For every "rule" about software engineering there is usually a very good reason but sometimes you have to actually experience it yourself first and learn the "hard way". I have built and designed many crappy systems that became a nightmare to understand, maintain and expand
, but each time I have learned a lot.