Procedurally generating islands

I’ve been curious about the approach used by this team: http://floatlands.net/

And i wanted to do something similar in Blender. Does anyone have any idea on how to start generating such geometry? Which kind of noise he could be using? And how does he “cut” the island?

for a normal everyday isl
ANT
https://2-t.imgbox.com/56Hon4Ta.jpg

or
use nodes to make a heightmap

https://4-t.imgbox.com/tfnh5P9z.jpg https://6-t.imgbox.com/xUbfHae9.jpg