What happens during an iPhone diagnostic?
The phone is tested function by function — display and touch, battery health and behaviour under load, charging and the port, speakers and microphones, cameras, Face ID, and the internal liquid indicators — so the actual fault is identified before any part is bought or fitted.
What gets checked
Display and touch across the whole panel, including ghost touch and dead strips. Battery health, cycle count and how the phone behaves under load, because unexpected shutdowns are usually a battery rather than a board. Charging behaviour and the state of the port and its flex. Speakers, microphones, both cameras, and Face ID.
If there is any suspicion of liquid exposure, the indicators and the board are inspected, because that changes what every other symptom means.
Why it saves money
Guessing costs more than checking. A phone that restarts randomly could be a battery or a board; a blank display could be the panel or the phone not booting at all. Fitting a screen to find out is an expensive way to learn.
After the inspection you get the fault, the repair that fixes it, and the price — and you decide from there. If the phone is not worth repairing, you will be told that rather than sold a part.