How to Diagnose Possible Dead Motherboard

"When I purchased my iFixit tool kit, my bf bought it and his name is here and can’t change it to me not him. I’m the one repairing phones, I’m Cay.

I was having this same problem with my granddaughters phone. It needed a new charging port #1 and #2 new screen because she broke the back and a piece of glass shard into the camera housing and next to the flex cable that goes to the screen. So none of that holding down buttons etc was going to work, idk until getting into the phone. Installed new screen, charging port, camera, and wireless charging pad and got the phone back on and process of backing up her 5k+ photos on the 32gb phone that was full with no sd card. It kept crashing. I put in an old 256 gb sd card and found out it’s not compatible and therefore wasn’t going to work. Maybe it would have, had I had the correct sd card, oh boy am I learning and idk if it’s been worth it. I couldn’t get it to stay on long enough to back up her photos to Google. Even if I could have Google’s storage would have froze for that amount of photos. So what I learned, charge the phone all the way first. Put in a large sd card second, so if it crashes and the operating system wants to update (is what happened when I held the volume up, home and power button, got the yellow triangle) maybe things would go to the sd card (idk, but probably not). Idk if it’s total trash or not now? Any help would be appreciated?"