Not so much an out-of-the-box tool, but I am using OpenAI's API to generate complete knowledge graphs for my own tool: Contextualise (a topic maps-based PKM tool). Although not perfect, this approach is working suprisingly well.

I am careful not to use it for actual learning - the information is not accurate enough to trust. But it can build a roadmap of details for you to research independently when you are new to a topic -- to fill in the "don't know what I don't know" level of knowledge. You can then independently research your knowledge gaps once they are identified.