Components
'Joomla\Component\<Component name>\Administrator\' points to administrator/components/com_<component name>/src
'Joomla\Component\<Component name>\Site\' points to components/com_<component name>/src
'Joomla\Component\<Component name>\Api\' points to api/components/com_<component name>/src
eg 'Joomla\Component\Content\Administrator' points to administrator/components/com_content/src
Modules
'Joomla\Module\<Module name>\Administrator\' points to administrator/modules/mod_<module name>/src
'Joomla\Module\<Module name>\Site\' points to modules/mod_<module name>/src
eg 'Joomla\Module\Login\Site' points to modules/mod_login/src
Plugins
'Joomla\Plugin\<Plugin type>\<Plugin name>\' points to plugins/<plugin type>/<plugin name>/src
eg 'Joomla\Plugin\Fields\Calendar' points to plugins/fields/calendar/src
Library Classes
'Joomla\CMS' points to libraries/src. Note that these are the classes which are described in the API docs on the Joomla CMS side.
'Joomla\SomethingElse' points to libraries/vendor/somethingelse/src.
eg 'Joomla\Event' points to libraries/vendor/event/src
If a library classname doesn't start with 'Joomla' then it's going to be found in one of the other directories under libraries/vendor/.