how can practice my knowledge in oop, data structure and design pattern to study new mobile app cross platform like xamarin

Stupid question, but I like programming and please help ... I studied lesson of: C ++ , pointers; c #; oop; datastructure and alogorithm; and design pattern. However, when I wanted to enter the mobile applications on xamarin, I did not benefit from what I studied previously. The question is how do I watch strucure of xamarin to understand how these many functions contain. Thanks and sorry because my English is not excellent.