How do I learn to build HMI and SCADA systems?

There is a real gap between using a screen someone else built and building it yourself. You are standing at that gap now, and it looks wide. It is not as wide as it feels. Everyone who builds these systems started exactly where you are, staring at a blank project and wondering how it all fits. Building is a skill you grow, not a talent you either have or lack. Start smaller than the whole system. Build one screen that controls one thing. Get it working. Then add the next piece. The people who seem to do this effortlessly are just further down a road you have now stepped onto. This week, take one small function and build the interface for it from scratch, even a rough one. Finishing something small teaches you more than reading about something big. Do that a few times and the locked door starts to open.