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

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.