I have no idea what is wrong with my iPhone 6s

Hello there, I need help with my iPhone 6s. Almost Everything in this phone has been replaced but I am still running into the issue of error nine when restoring on iTunes. The weird part is when I went to restore it in a cold environment it restored but then got stuck on a boot loop. Sometimes it gets further restored more than other times but usually ends up crashing or getting stuck in a boot loop. I keep getting error 21 and Error 9. Does anybody know why this might be?