Is the juce modules purely pre defined functions or does it contain objects also ?
What do you mean by “objects”? It has classes and functions for handling audio and UI. And you can create objects from those classes 
By objects I mean variables
Or arrays for creating waveform
Yes, you can do those things.
You should start by getting familiar with C++.
If you already are familiar with it, I recommend to look at the tutorials:
https://juce.com/learn/tutorials