Underground drawing tools function similarly to the aerial tools and can be designed on the same map as any aerial routes. By default, there is an underground toolset that allows you to draw underground paths to breakpoints, manholes, and handholes, and these can be connected back to any pole.
You'll notice the "Other" drawing pencil as well as the "Join" tool within the underground toolset, which allow you to draw and connect any node types as needed. If additional node types are required beyond the default options, you can update the dropdown for the node type attribute in the model editor.