It kinda sounds like the battery is not charging. Can you plug it into a USB ammeter to see if there is any current flow?
Otherwise, a bootlooping device can be caused by many things.
Try these things (if you haven't already) to see if anything helps:
- Try a hard reset (Home & Power button for 10 secs)
- Use a genuine Apple Lightning connector
- Use a genuine Apple charging brick and let it charge overnight
- Try to connect it to different computers (mac/pc if possible) with an updated iTunes
- Put the phone in recovery mode (search DFU Mode). Careful, this can reset your data
- Use 3uTools for putting it in and out of Recovery Mode
- Use 3uTools to flash the firmware (you can't downgrade)
- Check the condition of all the flexes (with magnification)
- Disconnect the Front Camera Ribbon from the logic board and reboot your phone
- Disconnect the screen assembly completely and see if iTunes will recognize the phone.
Otherwise, you may have a logic board issue. If you are uncomfortable doing these steps, consider finding a repair shop to do the troubleshooting. A shop that also does micro-soldering can go deeper if the problem is not modular in nature.