Apple unauthorized modifications replacement?

My phone is out of warranty and the screen broke so I thought I'd save money by replacing the screen myself, but that didn't work. So now I have to resort to apple out of warranty repair but realized that they don't accept iPhones that have been opened.
So my question is: how does apple know if I opened my phone and messed with the insides? I know it's wrong to try to trick apple but I'm poor and I need to fix my phone

What leads you to believe that they don't accept iPhones that have been opened? Apple doesn't turn down any opportunity to extract money from a customer.

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