What even is a "class method"?

what_even_is_a_class_method.rb

class A
  def self.b # we all agree this is a class method
    'A.b'
  end
end
A.b # => "A.b"

class << A
  def c # but what about this? it's the same thing, but we define it differently
    'A.c'
  end
end
A.c # => "A.c"

def A.d # Same thing here, this is equivalent to A.b and A.c
  'A.d'
end
A.d # => "A.d"

class Class
  def e # and what about this? it's not a singleton method, but you call it the same way
    'A.e'
  end
end
A.e # => "A.e"

A = Object.new
A.instance_eval do
  def self.f # and what about this? we define it the same as A.b, but self isn't a Class
    "A.f"
  end
end
A.f # => "A.f"
A # => #<Object:0x0000000152061618>