How can I access the class method and instance method in ruby?

In the first sentence you create anonymous class with superclass of String:

my_str.class.superclass # => String

But this is not the essence of your actual question :)

Instance is an object of some class: String.new() # creates instance of class String. Instances have classes (String.new()).class #=> String. All classes are actually instances of a class Class: String.class # => Class. Instances of Class class also have superclass - class that they inherit from.

Instance method is a method that you can call on instance of an object.

"st ri ng".split # split is an instance method of String class

Class method in Ruby is a common term for instance methods of an object of Class class (any class).

String.try_convert("abc") # try_convert is a class method of String class.

You can read more about instance and class methods in this article.