In Gimp only Gimp native format (something.xcf) is "Saved" Other formats including something.jpeg / something.jpg are "Exported" that is File -> Export see: https://docs.gimp.org/en/gimp-export-dialog.html
If you are starting with a jpeg then you can Export As and the filename extension remains jpeg.
If starting with some other file type, but want a jpg then File -> Export As and change the filename suffix to .jpg
If a new image, then the default is png. File -> Export and all you have to do is change the .png suffix to .jpg
If you always want a jpg and not a png then the default is changed in Edit -> Preferences -> Image Import & Export
If you are starting with a jpeg then you can Export As and the filename extension remains jpeg.
If starting with some other file type, but want a jpg then File -> Export As and change the filename suffix to .jpg
If a new image, then the default is png. File -> Export and all you have to do is change the .png suffix to .jpg
If you always want a jpg and not a png then the default is changed in Edit -> Preferences -> Image Import & Export