Why did this arduino burn out? SilentStepStick (TMC2130) stepper at 24V
I'm not entirely sure, but I think your 24V somehow ended up in the Arduino.
I would place the 24V rail and the 5V rail on opposite sides, that's why your breadboard should have one rail on each side. Sometimes they even come with 4 rails, indicated with a small gap in the lines on the side rails.

Technical schematics often use + and minus opposite to the way they work in reality. So these two grounded transformers can indeed create a potential of 19V across the 24V and the 5V connection. That's how you get 7V if you want to run a 12V fan silent/slower in a PC.