iPhone 5s screen blank

Hi! I'm looking for some technical advice regarding an iPhone 5S.

My device currently has a completely black screen, although it occasionally produces vibration sounds, suggesting that it might still be responding.

Here's its repair history:

- The original LCD was cracked but still displayed an image.

- The phone eventually developed a bootloop issue.

- I've tried replacing the LCD multiple times with different replacement screens, but none of them displayed anything.

- The battery has also been replaced.

- The device currently shows no visible Apple logo or display activity.

I'm wondering whether this could be related to the LCD connector, backlight circuit, display circuitry, or a motherboard-level issue.

I'd really appreciate any technical insight. I don't want to keep replacing screens without understanding the actual cause.

Thank you!