Game efficiency?

In my project I have stages, and as soon as the player enters another stage, the background changes and so do a whole bunch of other stuff. I’m wondering which is the best approach, efficiency-wise. My current options are to make those individual objects have a trigger of its own, or to create a world-wide variable, and then access that variable from a script which all the objects share.

So my question is, which would be more efficient? I feel like in this first project of mine, that’s always my question; “Whats more efficient?”. with the ease of Unity overall It’s easy for a beginner like me to learn how to do things, but I never know which executions are fast and which are slow

Would anyone know some document or something that more or less describes the efficiency of all the executions? (From rendenring, to destroying/creating objects in scripts…etc.)
Or is it something that one just learns over time through trial and error by using the Profiler and such?

Understanding and comparing how long different parts of your game take to execute and where performance bottlenecks are is exactly what the profiler is for. Unity - Manual: Profiler overview