OK When switched off there should be no looping as it it isn’t on.
If you remove the battery for 10 seconds you can ensure it is powered off
Replace battery ~ plug in charger ~ do not attempt to start phone ~ what happens?
After some time a quick press on the power key should show the battery charging without powering on the phone.
Check the battery by doing a spin test.
The charging electronics is on the main board and not a DIY option
You can dismantle the bottom module for a better clean, ensure the contacts are bright
In removing the display module check the water ingress indicators and other contacts, pogo pins and springs and their contacts.
Images of both the water indicator and the solder contact point can be seen in the following topic.