In Hippo CMS UI, the External Document Picker Base for RichText Field in CKEditor plugin is installed. Then you can configure a RichText (HTML) field(s) in bootstrap XML files (a.k.a Hippo CMS 'namespace'). So, when you are editing a document in CMS UI, you will see a button handled by this CKEditor plugin to select an external document in the RichText (HTML) editor field.
Here's how it works:
Therefore, in most cases, you don't have to care of the exdocpickerbase CKEditor plugin. You can simply add and configure it in your project and provide your own domain-specific REST service URL in the exdocpickerbase configuration in the custom CKEditor configuration. Then the exdocpickerbase CKEditor plugin will invoke your own domain-specific REST service URL to search your external documents.