What do rvm and rbenv do under the hood?

What do rvm and rbenv do under the hood?

For years, I've been dreaming about learning Ruby and the Rails framework.
(Most of my development career has left me too busy to devote time to
picking up a new language properly, but I'm making more time now.) I like
the notion of being able to work with Ruby to develop quickly, but I'm
having trouble understanding the Ruby installation process.
Each time I've encountered it, the Ruby installation process varies
slightly. The two laptops I've used over the past several years have
usually been running a then-recently released version of OS X, and all
seem to ship with Ruby 1.8.7. Any modern version of Rails requires Ruby
1.9.x. So, I search the web and invariable bump into a post like this:
Use this awesome tool called rvm to manage multiple Ruby installs on the
same machine.
or this:
rbenv is fantastic, and lightweight compared to rvm, use it instead.
Really? What is the Ruby install process doing, and why is it so
complicated? By now I've gotten Ruby installed and running, but it seems
like I've always had trouble with it. I'm genuinely confused and want to
understand how Ruby lives on my system. What files are these configuration
tools manipulating, and why can't I just do it by hand?
To put this a little differently: If I was RVM or rbevn, what steps am I
taking to make Ruby work on a given system? Am I manipulating
configuration files? Am I downloading source code and compiling it into an
interpreter? Am I downloading a precompiled interpreter?