yeah you pick something difficult.
But in your example it might be much simpler to generate the cube with enough subdivision.
I suggest you first look at the tools and from there see how you can use them to get to the result you want. Rather than look for the most obvious functionality to get to a result.
A lot of common tools are missing or made by other with nodegroups.
This topic got a lot of interesting resources : Let's regroup Geometry Nodes Groups - Resources and Links
Personally it took me like a mount to do just tests to learn the basic tools. For instances curves tend to be more versatile than mesh operations. I tend to use them much more than in regular modeling.
Have fun !