How to modify the output of a program for which you don’t have the source code

Is it ok not to understand how to solve the problems yourself, and use libraries instead?

In the general, no, it's not.

A library can save you the (hard!) work of figuring out how to solve a problem, and then debugging the solution, and then, maintaining it. But, if you're going to use it, you'd better make sure you understand how it works - why the solution actually solves the problem. You don't have to know how to invent cars, and engines, and robots that build car engines, if you're working as a mechanic - but you'd better understand how the parts work, what they all do, and how they fit together!

This is the reason why you will see many people become very specialized - many times only learning how to work with a single language, a single platform, a single framework and set of libraries.

That being said, there's only so much you will have time to learn. Sometimes you've got to take shortcuts - take them, but know they are shortcuts. Maybe you only read enough about a library to know you could figure it out, if you had the time. Or maybe you only figure out the two functions that you actually need to call, and only enough to make the calls correctly. That's a shortcut, which will come at a price - usually later, when someone (maybe an older, and more experienced, you) has to fix the code.

Activity diagrams don't use the concept of objects, as they usually don't show who does what, but only which actions/decisions are taken. You could use partitions (also called swim lanes) to indicate who executes the actions, but if you have more than two or three objects interacting, this can become rather unreadable.

For describing the interactions between several objects, it will be better to use a sequence diagram or a communication diagram (sometimes referred to by its old name of collaboration diagram).