What classes should be part of Business Logic Layer? [on hold]

I am very new to software architecture.I am developing an iPhone app. I have read that iOS follows MVC architecture, but one of my previous projects had busies logic layer, helper folder too in its project structure.

I am trying to understand what classes should go in Business logic folder and helper folder.

Is there any good link to understand how to design softwares/apps?

I want to know how software architecture is decided.