Apple Accuses me for Unauthorized Modification on my iPhone but i'm a layman

My iPhoneX abruptly stopped working and i went to the service center to get a solution. My phone is under warranty btw! Please note that on my first visit, they inspected my phone thoroughly and they did not find any kind of physical damage externally or internally.

After 7 days they come back saying some 'unauthorized modifications' have been done to my phone and when i asked them to specify what exactly they say they can't disclose it legally! This is highly disappointing and not justified.

As a layman i do not know how to modify my phone, this report makes absolutely no sense! I feel this deserves an explanation, if my phone will neither be replace and nor be repaired how are you resolving my issue?! I'm utterly frustrated and disappointed with the after sales service from a company of this stature.