Whether all VB.NET applications are implemented using layered approach
e.g. GUI,Business layer, Data Access layer and Back-end.

I come to know that at professional level, some projects are
implemented by using more than four layers, in that case how
application are written and what type of OO programming approach is
used.

This "question" is far from simple. You could (are) writen books about
this subject.

What exactly would you like to know?

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