how to make drop-down list where the items fetched from another spreadsheet ?
You will need to add that sheet as a table in the app. Then you can create a Ref column for the dropdown list.
but the problem on this solution that i dont want that table to be shown in the app, i need just a master data sheet
You just hide that view from app. Go to view for your master table which is currently visible on the app. And then select “ref” as position of that view.