Rails Context

Rails
1

Example: Belongs_to is according to the Rails Framework Documentation
a public instance method of Module
ActiveRecord::Associations::ClassMethods. David A. Black explains in
this book, that the instance method ends up as class method of
ActiveRecord::Base.

My general (beginner) question is:

  1. How do I know in which Rails context I can use which Rails instance
    methods? Is there any rule of thumb?

  2. How is the accessibility of the instance variables accross the MVC?
    How is it realized Rails internally?

  3. Any additional docu on that?

Thanks for Your guidance in advance