iPhone 12 suddenly turns off, not booting anymore

A friend of mine has an iPhone 12 that suddenly went black after he opened an app. He told me that it did fall down multiple times before that but that did not cause any issues immediately. I think that some internal components might be damaged but I don't know which. So I replaced the battery and the screen but its still not booting. I also went to a repair shop and they took a quick look inside and wired another battery and screen to it but with no success. They told me they could take a look at every part, but with 200$ that would be too expensive for us. Do you have any tips on how to find out what part is broken?