I have the following assumptions:

* TeX is a kind of language
* Plain TeX is the original environment for this language
* LaTeX (and other environments) is a set of compiled macros written using TeX primitives.
* Lets say MikTeX can organize those macros in local repository so I can reuse them in my LaTeX document.

But I am still not sure if CTAN contains packages for LaTeX or for TeX. Are they written using built-in LaTeX macros or written in pure TeX. If the second is true can I use those packages in plain TeX using `\input`? How should I organize macros from CTAN just download and put into the same folder with my document? Is there any package manager for plain TeX?