they can only rule on whatever's specifically put in front of them

Correct. Although you can also be an activist judge and make shit up like Jackson, Kagan and Sotomayor. You won't be respected for it as Gorsuch showed in this case (copied from YesMovement above):

The question at hand is, “Can a State force someone who provides her own expressive services to abandon her conscience and speak its preferred message instead?” Gorsuch said. “When the dissent finally gets around to that question— more than halfway into its opinion—it reimagines the facts of this case from top to bottom,” he wrote.