They don’t get very granular, but they give a fairly detailed overview of the methodology here:
https://www.consumerreports.org/cars/car-reliability-owner-satisfaction/who-makes-the-most-reliable-cars-a7824554938/
Edit: I used to subscribe, and as a result received their surveys occasionally. They seemed pretty well done, but people are people and probably aren’t always objective in their reporting.
They send a survey to their subscribers.
The next paragraph states: