Need advice as to where to begin with fixing my iPhone 6 Plus

Within this past week I have posted a couple questions I had during the rebuild of my iPhone 6 Plus, which I’d like to say the answers I received were the most helpful and thank you to those of whom helped me out, you definitely contributed to me being able to completely rebuild my device. Now that it’s put together I’ve come across a few other separate issues. To give a little background this is an iPhone 6 Plus that had completely stopped working and wouldn’t turn on about three years ago so it’s been sitting stored away for some time. If I remember correctly before it completely stopped working it was having some minimal ghost touch issues and the battery would get hot at times. So to fast forward a few years, last week I pulled it out, completely disassembled it along with another iPhone 6 Plus I bought for parts on eBay and began rebuilding it. Keep in mind that I am completely clueless as to what was wrong with the eBay phone or why it was being sold for parts. Most parts I used during the rebuild were from the eBay phone just because they looked to be in better condition except for the motherboard and the home button from my iPhone. Once I finished, I turned it on and it would flash with the Apple logo for a quick second and then the screen would just stay on a black screen. I tried to restart it and do a hard reset but got the same result, it would flash the Apple logo and the screen would just stay black. It also would show the dead battery image on the screen when it was completely dead and needed to be plugged in. Additionally, the battery would feel extremely hot after being on the charger for a short amount of time, I’d say about 30 minutes. I removed it from the charger and took a break for most of the day. Later that night I plugged it in again to charge for a bit because with it being on a black screen I couldn’t tell what the battery percentage was at and wanted to maybe connect it to my Mac and try to restore the settings or do a system recovery if it let me. Unfortunately, I fell asleep and a couple hours went by before I jumped up remembering the phone on the charger and it was almost too hot to touch. I let it cool down for a few hours and when I attempted to turn it on it wouldn’t turn on at all, not even a black screen. When I leave it plugged in the battery won’t even get warm, it’s just cold or room temperature. Whatever temperature that may be if it were powered off and just sitting on a desk. I’m sure that last incident blew out or killed the battery along with other components I’m not educated enough to know exactly which parts. So at this point I know I’ve done something to have to replace the battery and any other parts due to overheating it along with whatever possible reasons as to why it wouldn’t turn on prior to that. If anyone has any help to provide me with or guidance to direct me in the right direction I would be incredibly grateful and appreciate any suggestions or advice, because at this point I don’t even know where to begin or have the slightest idea which parts would be the problem parts.

Again, I can’t thank you enough for your time and consideration with helping me through this process. Thank you!