There are absolutely engineers (with or without PE licenses, I’m not sure if that’s necessary in aerospace) signing off on airplanes, even after they’re built. For a mechanic to make a major modification or repair to an airframe or power plant it needs to be approved somehow, either covered by the airplane’s original Type Certificate and detailed in the maintenance manual, covered in a supplemental type certificate (STC), or they can work with an aeronautical engineer to design and approve the repair. I have forgotten the exact line between “you can get an A&P IA to approve this skin patch plate” and “You need to either call an AE to design a repair for this but it might be cheaper to buy a whole wing.”