A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
The answer depends on whether you are using the free online version of Microsoft 365, in which case your files are being saved to your OneDrive, or if you are using a paid subscription to Microsoft 365, in which case your files are saved by default to your OneDrive unless you chose to save them to your hard drive by opening any app and going to File > Options > Save.