Trying to understand rotation of parented objects (relative rotation?) I'm trying to understand how Blender applies parent-child relationship w.r.t rotation. My end goal is to get the child rotation relative to the world. While there might be a function for that, I'd still want to understand how things work.
To make things simple, I'll use the X/Z ortho view, and only rotate on Y. Here's my basic setup:

The object at the center (called 'Bottom' has a Y rotation of 135°). The other object ('Top') has a Y rotation of 45°:

Now I make 'Bottom' the parent of 'Top', and 'Top' still has a 45° rotation:

Finally, I rotate 'Bottom' by 45° and get the following result:

Now, Bottom's rotation is 180° (not shown in the screenshot), which seems reasonable, and Top's rotation is still 45°.
While it's obvious Top's rotation shouldn't change if I rotate Bottom, it's not obvious what's Top's rotation is relative to. Clearly, its parent is vertical and it is horizontal, so where did that 45° come from?
When Top became a child of Bottom, its rotation became (in practice) relative to Bottom's, why didn't the numbers change? How is the global/world rotation is calculated based on the parent and child rotations?
A simple way is to just use Clear and Keep Transformation (Alt P) on the child to visualize the final rotation (global/world). As soon as you check the value, just press Ctrl Z to UNDO the Clear Parent.
The numbers of Top (child object) do not change because they exhibit rotation without the influence of the parent. To update the values, make sure the child inherits the rotation from the parent with Alt P.