How does the emacs load the package?

Question

I'm always curious about the loading process of the package in Emacs. You know, Some packages are builtin packages, and some are third-party packages.
For example, you can install the "auto-complete" package from MELPA or MELPA-STABLE, or you can get the source from github .
If you install the package from MELPA, you will find the package files in "~/.emacs.d/elpa/auto-complete***".

auto-complete-autoloads.el  
auto-complete-config.el  
auto-complete-config.elc  
auto-complete.el  
auto-complete.elc  
auto-complete-pkg.el  
dict;;it's a directory

What does the emacs do for us to install the package? It just downloads all the files from MELPA and puts them in "~/.emacs.d/elpa/auto-complete***"? How does the emacs know the existense of the auto-complete-package and load them when you start emacs next time?
I know I should add something to "~/.emacs.d/init.el" if get the source from github. Maybe like this

(add-to-list 'load-path "path-to-download-folder/auto-complete")
(require 'auto-complete-config)
(ac-config-default)

But I can't see any changes in "init.el" if I install the package from MELPA. Maybe the emacs assign the path of the auto-complete to the variable load-path. But then?
How does the emacs load the package? How does it know my "ALT-x + command"?
And I really don't know the relationship of these files the emacs downloaded from MELPA and their function. Could anyone tell me the whole loading process of a package? What does the emacs do behind us to install the package from MELPA/MELPA-STABLE?

Answer 1

Emacs (by default) automatically loads (almost) all the files of the form ~/.emacs.d/elpa/*/*-autoloads.el . The package's <pkg>-autoloads.el will contain more or less the equivalent of your "add-to-list + require + ...".