Wikilinks are the internal links that connect the files in your knowledge base.
To create a wikilink use [[ and type the name of another file in your repo, for example [graph-visualization]
You can also create a [[placeholder]]. A placeholder is a wikilink that doesn't have a target file and a link to a placeholder is styled differently so you can easily tell them apart. They can still be helpful to highlight connections.
Open the graph with Foam: Show Graph command, and look at the placeholder node.
Remember, with CTRL/CMD+click on a wikilink you can navigate to the note, or create it (if the link is a placeholder).