There are details that are not there so its a little difficult to give you exact steps, such as:
* When does this run?
* Who is it triggered?
* Where is the massive Excel stored?
* Where is the Excel stored that you want the partial data put into?
Essentially (setting aside I do not know some things)
You will have to make sure there is an Excel Table around the data in the big excel, or you have to use Office Scripts to get the data out.
Either case you would end up building a Flow, or in Copilot land probably an Agent Flow instead that will take either trigger based on some Time based schedule, or when a file is updated or created. Since you said its autonomous there would not be a user to upload a document to the agent.
So you would need to provide step by step details of what you want to do.
1. Trigger the Flow somehow
2. Point at the bigger excel
--Use either Graph API, Excel Actions or Excel Scripts to get the data
3. Iterate through the data and inside that, add the data to the new Excel Sheet/Workbook, which also needs either Graph AI, An Excel Table, or Excel Scripts where you would pass data into the new excel
The actual flow will depend on factors such as the questions above.
Set aside copilot for a moment, your work will be done in a flow, whether its a Copilot Agent Flow directly or a PA Flow you build and trigger, they are both PA Flows, one is simply built in the CoPilot Ux
Since its autonomous you need to know how to trigger the Agent / Trigger which will the process of processing the data into the new file
You will need to know where the file will go, where the master file is etc etc.
Then put the steps together based on these answers
which if you share, we can help you better