Bootsnap is a library that overrides Kernel#require, Kernel#load, Module#autoload and in the case that ActiveSupport is used, it will also override a number of ActiveSupport methods.
Bootsnap creates 2 kinds of caches, a stable, long lived cache out of Ruby and Gem directories. These are assumed to never change and so we can cache more aggresively. Application code is expected to change frequently, so it is cached with little aggression (short lived bursts that should last only as long as the app takes to boot). This is the “volatile” cache.
Below is a diagram explaining how the overrides work.
In this diagram, you might notice that we refer to cache and autoload_path_cache as the main points of override. These are calculated using the concepts described in Caching Paths.