Packages are a language-level concept in Python. A package may be a collection of source files on the filesystem, it's still a package, except that doesn't lend itself well to distribution. Moreover, from the language standpoint, the usage of a package (like "import numpy") is the same on all architectures, but the files needed for distribution (native extension module binaries) may differ.

Python is rife with Monty Python references, such as "spam and eggs" in place of "foo and bar", so "egg" was a natural choice.

The linked page has hyperlinks that you can click to get to the specification for at least Wheels.

Pokemon are involved it gets a little confusing.