mounting powerbook as Target from Powerbookair 2017

Sadly you have that just about right my friend... except File Sharing via Wifi...

Set up file sharing on Mac - Apple Support

USB won't work as far as I know, Firewire or Thunderbolt only.

See if the MBPro is seen within Disk Utility on the MBAir. This will tell you whether the connection is working. You should be able to use USB since neither Mac is running macOS 11.x+. The Thunderbolt cable requirement is only if that computer is running macOS 11.x+. See:

Transfer files between two Mac computers using target disk mode - Apple Support

Note: If either of the computers has macOS 11 or later installed, you must connect the two computers using a Thunderbolt cable.

Is Sierra up to date? If not, then you need to install all macOS updates for Sierra since was only patched later on with support for APFS volumes.

I need to see the Mojave hard drive in the Sierra powerbook-air. The Mojave Powerbook (2013) has 4 Thunderbolt and 4USBA. The powerbookAir has 2 USB-A only so my only option is to copy the whole internal drive (user) in an external drive and then open it up with the powerbookAir.

I understand I have no options to boot it as Target disk.

Transfer files between two computers using target disk mode

The Oldest version in the Target Disk Mode support document is High Sierra macOS 10.13

Startup CP in Mojave shows ...