Is there a way to use functions defined in a library in any class?
Yes, there is a way.

However that's not how object oriented programming (OOP) is supposed to be used, at least in theory.

Most likely you won't like my advice, but here it is: take it easy, find a good book or online course and learn to program on a system that is simpler to debug, than a microcontroller.

Your PC, for example.

C is closer to the HW than java. C++ can be seen as both of them, but there is not so much you can do without learning C, properly.
From that perspective, C++ can be seen also as a superset of C, only much more complex.

Have a look at this explanation, for example.
It's quite simple, but exhaustive. Read it carefully and you will find out that pointers are not so difficult to grasp, after all.

If you spend enough time to get the basics right, it will save you lots of frustration (and, yes, time!) later on.

For a more systematic approach, try this.