Once a new derived image is created via the 'Edit' page of the Media Library interface and you're ready to save this verison, you should click on the generated link below the preview panel. This will trigger the requested transformation and it will open the edited image in your browser tab.
This creates a new derived/transformed version that is linked to the original image.
You can view all derived/transformed resources of the original image by clicking on the "view derived images" button on the 'Edit' page of the image.
The below video demonstrates this: