“Entirely different” is a bit of a stretch. It’s about as close as you can get in this world without relabeling it. It’s built on the same architecture, same manufacturing processes, its just a different number of cores with different priorities for phone efficiency.
When Apple piloted the switch to Apple silicon, it sent developers a Mac mini with an iPhone chip in it before they released the M1. This is that exact same thing, but newer.

For sure, but I bet some work can be done to allow it to work on the A18 Pro down the line. Maybe.