Your gimp .xcf file keeps much more information that other formats. Keep your .xcf safe.
For other simpler formats use the Export (or Export-As) command (1) see: https://docs.gimp.org/3.0/fr/gimp-export-dialog.html
The default Export format for Gimp 3 is .png (2) but you can select other formats using the drop down menu (3)
That gives options (4) for the file type chosen. This one .jpg you set quality .....
NOTE: An image with layers is flattened to a single layer. When the layer has transperancy use png. Use a jpeg where transparency is not required.
For other simpler formats use the Export (or Export-As) command (1) see: https://docs.gimp.org/3.0/fr/gimp-export-dialog.html
The default Export format for Gimp 3 is .png (2) but you can select other formats using the drop down menu (3)
That gives options (4) for the file type chosen. This one .jpg you set quality .....
NOTE: An image with layers is flattened to a single layer. When the layer has transperancy use png. Use a jpeg where transparency is not required.