Thank you so much! I'm really glad you enjoyed it.
Everything is procedurally generated except for maybe the font, when typing in the names :P
The planet surfaces is what I spent most of my time on working. It uses open simplex noise, "domain warping" and a little spice that I came up with myself to get some more heterogeneity into the generation.
Basically in different parts of the terrain, different settings get plugged into the noise function. Here's a little toy version you can play with, if you are interested: