Libreoffice Writer is a word processor, where a single file represents a single document. To create reference notes, Writer has the “Comments” feature. If you wish to make more complex and longer “notes” available, you could create these in another Writer document, and then, using Hyperlinks, link to it from within the main document, or from within the comments. Yes, you can also create hyperlinks in the Comments.
Depending on your user case, using hidden sections are another possibility to include information that should not be visible or printed any time. You can set a variable and have these sections shown or hidden based on the value of that variable. Thus, you can show or hide sections by changing a single variable.
This all gives plenty of flexibility to connect related information to a document, or to include information that at times needs to be hidden.