is it possible to complete all the courses on this app, are there hidden levels

Answer 1

Many have already completed all the 13 courses, it's not really hard. But completling them does not mean you're an expert on them, they're just the basics. There is way more things to learn than what SoloLearn teaches.

Answer 2

Yes. All of the courses available can be completed on the SoloLearn app for free. I'm not sure what you mean by "hidden levels," but if I had to answer, no there are not.