This is going to require us to look at how the GetItemAtIndex node anticipates and handles input structure. The list input obviously requires a list structure and the index input is expecting an integer. Both inputs can have additional structure beyond the basic format, but that doesn’t change how the node handles their relationship.
In the first example, you can see that
index structure doesn’t change where the node is looking for the index within the list structure, only the structure it returns.
Even with that additional level of data in the list structure, the node treats the top level list as the input. This is obviously where list levels can change what level the node iterates over, but list levels can’t change where the index is looking because the node itself doesn’t use structure to identify location (the level to process index).
This brings us to something called Replication Guides. Replication guides are an additional level of control (similar to list levels but even more specific) that we can use with DesignScript. They essentially allow you to specify the exact order of input levels to replicate over. Read the guide in the link to get more information and see how they work.