It seems to me that we have to absorb a bunch of stuff, forget most of it, revisit it and get new insights, and do this over and over again; and then mull over our own particular take on the subject until we finally come up with an original thought that is a valuable synthesis of everything we’ve learned plus our own unique views on the world.
But sometimes a new idea seems to come from "nowhere".
I like this from the Chapter "Having One’s Own Way of Seeing" from Grammar for a Full Life by Lawrence Weinstein:
My former colleague David Honick one told me of the fellow graduate student who, fifty years before, had had a study carrel - an assigned desk and shelves - next to his in Harvard’s huge Widener Library. David’s own carrel was plentifully stocked with books he was consulting for his dissertation, but his neighbor’s was empty, a student’s bare cupboard, and the neighbor himself arrived there with only a blank lined yellow pad in hand, where he’d write thoughts down, apparently out of the blue. When, eventually, David asked his name, the msyterious abutter replied, "Noam Chomsky."