Toy3D is a simple 3D engine written using Apple's Metal framework. It's purpose is to show how to get started using Metal in your first application, it is not intended to be used by anyone for anything serious :)
I would recommend reading this blog post first, it covers all of the basic Metal concepts and the creation of all the classes in this library: http://markdaws.net/blog/2019-12-17-toy3d
There is an example project that uses the framework here: https://github.com/markdaws/metal-example
NOTE: The metal shaders used in the basic Material class can be found in the metal-example project above. There doesn't seem to be a way to package the shaders as part of a Swift package at this time.