Todo List Project: how does Ruby/Rails know each todo list item is an instance of the TodoList Class?

A huge theme in the Todo List project has been test-driven programming. We have written quite a few tests ensuring things like "title" and "description" for each todo list exist, and are not too short.

The requirements for the todo lists are defined by the TodoList Class in the todo_list.rb file (odot/app/models/todo_list.rb). The code looks like this:

class TodoList < ActiveRecord::Base
    validates :title, presence: true
    validates :title, length: { minimum: 3 }
    validates :description, presence: true
    validates :description, length: { minimum: 5 }
end

Simple enough. However, what I don't understand is how Ruby/Rails knows that each todo list item created on Rails--by navigating to localhost:3000/todo_lists and clicking "New Todo list" etc--is an instance of the TodoList Class. I don't remember writing any code to the effect of

when click_button("New Todo list")
    new_todo = TodoList.new(title, description)
end

And since we haven't done that, I don't know how the tests we are writing are working, or how Rails knows to throw an error if one fails. Like, for example, we said that title length is a minimum of 3, but how is it that Ruby/Rails knows that filling in the Title box on Rails is defining the title for an instance of the TodoList class? There isn't any code (that I know of) connecting the two.