iPhone 6 plus keeps restaring every single minute

What were you doing just before it started bootlooping? I had a customer suffer something similar and they had a filled-to-the-rim memory and tried updating. It went into bootlooping and nothing would make it work.

Here are some things to try:

• 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)
• 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.