It does kinda beg for a better UI. I’m not sure what or how, but it took me a long time to really “get” what was going on here as well.
Here are my cribsheets on this stuff:
Unity button onclick function script callback gameobject notes:
https://discussions.unity.com/t/741309/6
For sliders and dropdowns and other value-returning UI elements:
https://discussions.unity.com/t/855879/2
And passing “more interesting” things when buttons are pressed: