I see this all the time. Its real simple people. You flesh out an idea, a concept, a thought. The phrase comes from having a “skeleton” of an idea. Then you “put flesh on the bones” to create a full idea. That’s where it comes from. Real simple.
Flush out means to chase something into the open. A posse on a manhunt would want to “flush out” their quarry. A bird hunter would try to “flush out” a pheasant. You also “flush out” a wound or your eye when you get something in it. See the difference? Good, now quit screwing it up. Bastards.
By the way, going to try to post every day. Enjoy.