How do you link from inside content to another content item?

From Joomla! Documentation

The simple answer is that you get the page URL you want to link to and then make a link using whatever text editor you are using or (if you have no wysiwyg) with html.

The more complicated answer is that some text editors have fancier links managers. A number of these editors are available in the Joomla! Extensions Directory.

See the Tips and Tricks article How_to_control_module_display_when_linking_to_an_article_with_no_menu_item for more detailed information.