How do I understand complex building systems that interact?

You understand most building systems on their own now, and what stumps you is how they behave together, where heating, lighting, and the building's shell pull against each other. That is not a gap in your knowledge. It is the genuinely hardest part of the field, because interactions do not add up simply. They multiply, and one change ripples everywhere. The way through is to stop studying each system in isolation and start tracing the connections. When you change one thing, ask what else moves because of it. A tighter shell changes the heating load, which changes the equipment, which changes the cost. Follow one thread all the way through, and the tangle slowly becomes a pattern you can see. Take one real building and pick a single change, then trace every other system it touches, one link at a time, on paper. Do that a few times and the interactions stop feeling like chaos and start feeling like cause and effect. Complex systems are not beyond you. They just have to be understood as a web, not a list, and webs are learned one strand at a time.