Is this a battery or lcd problem?

Bare with me haha.

So I am getting into fixing iPhones, and I started practicing on an old iPhone 6. I’m pretty sure the battery has been replaced bc it doesn’t have an Apple logo. Anyways, when I was practicing reconnecting the display, it showed glitched white vertical lines. I simple reseated the lcd cables and that fixed it. But the next time I reconnected the screen, the same white bars showed up, the only difference was no matter how many times I replugged the cables, it was the same glitches every time. I think the screen cables have just worn out as this phone looks to have been opened a lot. One of the times I reconnect the lcd it was completely black, and I plugged it in. When I did so it buzzed when connected to power (remember that) but still was glitched when I reseated the cables. I did some research and found that you are supposed to unplug the battery when replacing the screen, so I did. I installed the lcd cables and then reconnected the battery, but now there is nothing. The mother board definitely feels warm as usual, but the lcd is plain black with no backlight and when I plug in the phone it does not vibrate at all. Please let me know if any of you have a solution. Do you think it’s just the LCD that needs replacing?