How to re-create the structure in this excellent 3d random minesweeper?

I want to recreate this mobile game in Unity

https://play.google.com/store/apps/details?id=com.kazutani.DRsweeper

Youtube video

I have some basic experience with Unity and C#. I have an idea how to do the game mechanics and UI, but I have no idea how to create the whole structure. I see it is procedurally generated, that it avoids linking more distant nodes and that most of the nodes are not within the structure, but gravitate towards the outside of the structure, but that's as far as I can go.

Any pointers on how to generate such a structure would be very helpful. Thanks in advance.