This is a very similar idea to Gary Bernhardt's: Functional Core, Functional Core, Imperative Shell: https://www.destroyallsoftware.com/screencasts/catalog/funct....

I prefer Gary's explanation of it because he jumps right into the meat of the problem and shows code that models this architecture. The video doesn't require you to know Ruby to understand what he is saying, just as long as you know some basic testing phrases; Mocking, Stubbing, Etc, you should be able to follow along.