How do I swap Rails components with my own? How to swap Rails components in and out?

Question

As far as I understand, what defines a Rails component/module is that it subclasses an own Railtie class from Rails::Railtie (the latter being an interface and being unable of being instantiated.)

How do the components/modules get loaded and how do I swap for example ActiveRecord with my own ORM-module?

Yours,

von Spotz