Let me give an example to help you understand how the data flow.
Let’s take your example - I create one folder, fetch 5 files from an API, and I want to upload it to Google Drive. My first Google Drive node (creates folder), returns a single item (since I created a single folder). My next node, HTTP Request node, fetches 5 files from a URL. Now, we have 5 input items for the next node. Since we have only one Parent ID and 5 files, n8n will only be able to use the Parent ID once with the expression that you’re using. To let n8n know that it should use the same Parent ID for all the files, you need to use the expression
I hope this clears your doubt a bit. Let me know if you have further questions.