Concept maps are a very useful tool for retrieval practice, metacognition and developing deeper connections and elaborate understanding in a subject domain.

A great set of tools can be found at https://cmap.ihmc.us/cmaptools/

The software is free, works on many different systems, and can be stored in the cloud so they can be edited and embedded in web pages and exported in many different formats. Concept maps can be linked together to give many levels of detail.