Adobe coded in an easy way to reference other layers and properties.
It's called the Pickwhip. You may have seen it before in the context of layer parenting.
What the Pickwhip does is lets you visually select which layer / property you want to reference in the expression by dragging a line straight towards it.
Once you let go of your mouse, an address leading to the value of that property will show up in your code.
Reference values from other properties
We can use the Pickwhip to reference the value of a slider in our expression, which will give us a number that we can use to control something else.
Reference properties from other comps
By opening multiple lines you can use the Pickwhip to select a property from a different comp.
While we are at it, I once made a song about parenting in After-Effects.