Indoor portrait photography, why is the background white?

The reason why the background is white is because the light shines too brightly on the background. Consider keeping the light away from the background.

The flash on the fuselage has automatic adjustment function, so don't worry.

If you use your own flash, you should experiment more and sum up more experience. Generally, the best combination of aperture and shutter can be found by taking a few more photos with the M file of the camera.