I build them.
I can’t say how tempting it is to end there, but I won’t.
This is one of those perennial questions that all writers get and it’s surprisingly hard to answer in a way that satisfies both the person who asked and yourself. But here goes.
The initial fragment of an idea for one of my stories or novels could come from anywhere: old research, a dream, a conversation at a con, two apparently unrelated words clicking together in my head. But that’s not a story, that’s a starting point. The real work happens when I take that moment, whatever it is, and start sticking bits onto it and asking, “well yes, but then what happens?” Or, “all right, that’s cool, what else can I throw into the stew?”
Goblin + laptop isn’t a story, but it popped up in my head as an interesting combination of concepts. Make it a laptop that becomes a goblin and is the familiar of a sorcerer and you start to get there. Add that the sorcerer is a hacker who uses code to cast spells and combine it with a parallel worlds story where the worlds are accessed as you would webpages, using the medium of the goblin/laptop and you have the seed of the WebMage books.