A lot of confusion comes down to some bad use of inheritance. Probably the most important OO concept is the Liskov Substitution Principal, viz that if you specify a superclass in a certain place, then any subclass should be a valid use there. The strategy pattern (or its hyperpituitary cousin, IoC) takes advantage of this to allow for run-time change of behaviors, e.g., being able to choose through configuration how to serialize a data structure so the same basic logic can output, say, a dataset as a table or a graph. That said, as OO architecture has matured over the last few decades, there has definitely been a tendency to favor composition over inheritance for code flexibility.

I would recommend Head-First Design Patterns as another great book for being able to “get” object-oriented architecture.