It's not CSV, it's JSON. You get an array of objects, where each object is a JSON element with key:value pairs. The keys represent the column names in Excel.
So, if you want to get the "ItemInternalId" value of the first element, use %GetItemsResponse.value[0]['ItemInternalId']%. The 0 in there indicates the item index in the list. It's 0-based, so you need to use 0 to get the first item. And then you can just use a column name to get a specific value.
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