The gem isn't freezing instances of the objects, but rather the classes themselves. In Ruby (nearly) everything is an object, so for example Array is an object of type Class. Classes are also open by defualt, allowing them to be altered at run time. There's nothing stopping me doing the following in my app:

  irb(main):001* class Array
  irb(main):002*   def hello
  irb(main):003*     puts "hello"
  irb(main):004*   end
  irb(main):005> end
  => :hello
  irb(main):006> [].hello
  hello

This sort of monkey patching is used a lot in the ActiveSupport gem to give convenient methods such as:

  irb(main):001:0> 3.days.ago
  => Sat, 28 Dec 2024 13:10:24.562785251 GMT +00:00

Integer is extented with the `days` method making date maths very intuative.

This gem freezes the core classes, essentially closing them, so doing the above raises an error:

  irb(main):007> require 'refrigerator'
  => true
  irb(main):008> Refrigerator.freeze_core
  => nil
  irb(main):009* class Array
  irb(main):010*   def hello
  irb(main):011*     puts "hello"
  irb(main):012*   end
  irb(main):013> end
  (irb):10:in `<class:Array>': can't modify frozen class: Array (FrozenError)

Looking at the code in the gem, all it's doing is calling https://apidock.com/ruby/Object/freeze on all call modules. The frozen flag is an inbuilt part of the language and as far as I'm aware has a performance benefit. In fact the major verison of Ruby will have all string instances frozen by default.