To make Notepad files always open in a new or separate window in Windows 11, open Notepad setting and change the 'Opening files' option.

Side note: Microsoft does allow you to change to any app, by right-clicking a relevant file and choosing open with>Choose default app.

File Setup To build a CSV file to open in Microsoft Excel, you'll need to decide how many columns you'll use in Microsoft Excel for the categories of information you're preparing in Notepad.