I talked about cad, machining, wiring, and plc programming. First I prefaced my answer with I was no where near mastery of this because people had done this for 20 years and still said they were learning, so if anything I was proficient at most. Then I used my work project as an example, where I had to build a tape feeder and cutter that would work with various locating pins. I talked about how I had to prototype a design, design it on cad, machine it on a cnc mill and lathe, wire it, and then write the program on an PLC, and all while troubleshooting any problems that came along.