@mel8729 anything like that is hard for us to verify since we do not have direct access to the phone. I suggest that you get a USB-C ammeter and see how many amps your phone pulls from the charger when it is turned on. That way you have an idea if it does do anything. Next, I would consider replacing the charging port as well as the battery (if it is still the original battery) Then re-evaluate to see if your phone now charges and consumes power. Now you can work toward the motherboard since you now know that it is getting power.
Since iFixit does not have a specific guide for this, use something like [https://www.youtube.com/watch?v=LZ-M2WWf2e0|this video] to work on your phone. A replacement charging port is readily available at many online stores.
While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has a device with the same issues as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this. Repair is War on Entropy!