Godot currently cannot be compiled and used as a library, so the ways for unity to use something out of godot, without making changes to the engine, is using common inter process comunication techniques: You could make a script for godot that listens and responds to requests on a network port, or you could execute it from the command line and capture the process standard output or use some file in the filesystem as means for data exchange.
On the other hand, Unity can be used as a library (even though I don’t know any detail beyond this statement):
It probably is a C++ library, not a .net one, so you would have to use gdnative/gdextension, create a module or change the godot engine in order to use unity as a library for something.
Sounds like Godot → Unity (probably No, but can use with side-work) / Unity → Godot (there is some hard way, Yes)
self-network-communication and running with command line is very interesting ways, thank you for your ideas