[sergiotapia]

Blame! is a manga where in the future humans have robots that build, and are controlled by people with Net Terminal Genes. Something happens and those humans die leaving the robots building non-stop procedurally for eons. By the time our protagonist moves about in the world, its said the Megastructure reaches from Earth all the way to Jupiter.

Also, the movie Fracture features these cool marble machines. https://www.youtube.com/watch?v=Y-v6E9H6nh0

Back when movies were made with unique good scripts and not marvel slop.

[stavros]

Where did they get the material for it?

[sergiotapia]

They never really specify beyond the planets themselves being consumed wholesale. Some of the structures are just hollow spheres: https://preview.redd.it/7tvkbj5bp2hb1.jpg?width=1951&format=...

Also conceivable that meteors etc crash into the megastructure providing it with endless resources.