Apple Repair destroyed my iPhone two times in a row

I bought an iPhone 12 Pro from a local friend some months ago. It had some minor scratches in the display, nothing to worry about.

Because the device was only 2.5 months old and he didn't know of any repairments done in the past, it should have been under guarantee till August this year.

However, last month some piece in the iPhone rattled when I shaked it, so I sent it to the next repair center. I receive it back and it was unrepaired because of "not authorized modifications".

The support couldn't tell me what exactly should have been modified and to my surprise, the iPhone did not boot anymore (boot loop, recovery via my Mac not possible: error 4013).

After some hard times, the support agreed on inspecting my device again. I sent it to repair center again, the again decline it, and after it came back, something has changed: The rattle was away!

But instead, I still can not boot my device and my power button is broken (I just can not press it, its flush with the device).

What can I do now? I understand that maybe, maybe my friend really lies, but anyway I think is just stupid that the Apple Repair Center breaks my whole device because of that.

They also removed the guarantee from the iPhone.