Drupal multisites share modules and libraries?
Yes.

In a multisite setup anything in sites/all is shared.
This means sites/all/modules, sites/all/themes, sites/all/libraries are all shared.

In addition to that, if you have things that are specific to a subsite, for example, a sub-theme, or a module or library, you can add them in sites/subsite_name/modules, sites/subsite_name/themes, sites/subsite_name/libraries.