MVC is the first thing anyone learns about Rails and the last thing people actually get right. The pattern itself is simple. Three layers, each with one job. Where it falls apart is the stuff that doesn't fit cleanly into any of the three, and that's exactly what interviewers poke at.
I'll walk through the three layers fast, then spend the real time on the question that separates a junior answer from a mid-level one: where do you put the logic for creating an order, charging Stripe, and sending a confirmation email?
The three layers
The model owns your data and the rules attached to it. With Active Record that means associations, validations, scopes, and small bits of behavior that belong to the record itself, like order.total or user.full_name. If the question is "what is true about this record," the answer lives in the model.
The controller handles one request. It reads params, checks who you are and what you're allowed to do, calls a model or a service, and picks a response. That's it. A controller should read like a list of instructions a manager gives, not like the work itself. People call a bloated one a "fat controller," and it's a smell.
