No point on giving up on this device so quickly...it is an iPhone 7 after all. At the very least, bring it to a reputable micro-solderer to see if they can find and fix the issue. Boot-loopers are tough and can have deep problems and aren't always repairable but most micro-soldering repair shops work on a no fix/no fee basis so there's not much risk involved.
Can you plug it into a USB ammeter to see if there is any current flow? Otherwise, many things can cause a boot-looping device. Give us more history by commenting on this answer.
A few things to try first if you haven't already:
- 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.