SDL Web 8 (including 8.5) currently does not support adding anything other than images directly into a Rich Text field. Adding support for YouTube videos can however be done through using an ECL provider (https://www.sdltridionworld.com/community/2011_extensions/youtube_ecl_provider.aspx).
In the Experience Manager view, the source view of a Rich text field has deliberately been removed, to keep the interface simple. Once in the field, you can switch in the ribbon bar to Home, and choose the Open in Form View button, which will open the Component in the full featured view, which will allow you to edit the source of the Rich Text field among other things.