iPhone will not complete boot up

its bootlooping.

1.battery is disconnected or has other issue

2.issue with the nand or other chip

3.water damage

try putting it in recovery mode and restoring (or update but if it doesn't work restore) the iPhone using iTunes or finder(newer macs)

If you see the Apple logo or a red or blue screen during startup, try these steps:

1. Connect your device to a computer. On a Mac with macOS Catalina 10.15, open Finder. On a Mac with macOS Mojave 10.14 or earlier, or on a PC, open iTunes.
2. Locate your device on your computer.
3. While your device is connected, follow these step: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then, press and hold the Side button until you see the recovery mode screen.
4. When you get the option to restore or update, choose Update. Your computer will try to reinstall iOS or iPadOS without erasing your data.

5.Your computer will download the software for your device. If it takes more than 15 minutes, your device will exit recovery mode and you'll need to repeat steps 2 and 3.