Using GIMP, four or so filters, and combining different elements.
Oh, and just to "troll" a bit, I applied the Hue and Contrast effect from PhotoMash.
I think as an end type effect applied to our own creations and/or edited materials with proper reference is perfectly acceptable.
it depends on if the imagery is Public Domain, CC-BY, or used within the realms of Fair Use.
I made it based of off this "City on the Ocean" from Artbreeder (https://www.artbreeder.com/i?k=31ac7b8df714199c519f6952)
