I am trying to turn an old phone into a portable Raspberry Pi. I want to wire a Raspberry Pi Zero W into it's display and battery. Is this possible? If so, how do I do it?
If your old phone is an android
Step 1. Turn on the wifi hot spot on your Android phone.
Step 2. Connect the raspberry pi to the phone's wifi network.
Step 3. Set up the VNC as if you were connected to a regular router.
The phone and the pi are on the same wifi network so can communicate without even having any internet access. Credit googleverse
If you have an apple iphone here's a slightly more complicated process Credit Being Engineers
As for wiring it into your phone battery it may be easier to get a LiPo Battery HAT Although I'll admit i'm not very knowledgeable about this aspect, and I cannot just comment this (not enough reputation).