I still need to learn more about how Godot is using ray marching, but FWIW there is a lot to learn on ShaderToy and IQ's articles about the basic techniques.
https://www.iquilezles.org/www/index.htm
https://www.shadertoy.com/view/Xds3zN
The Media Molecule team did a pretty amazing job integrating ray marching into a game pipeline and presented it at Siggraph
http://advances.realtimerendering.com/s2015/AlexEvans_SIGGRA...