The reflective umbrella for photography can reflect the softer lighting effect, right? Why?

Yes, this is because the reflective umbrella converts the electric light source of the flashlight into a surface light source. Under the same illumination, the light emitting area of the light source is larger, and due to the diffraction effect of light, it produces a surround lighting effect on the subject.

Reflective umbrellas are usually used with off-board flashlights. Compared with the subject, the flash is small in size and extremely high in brightness, which can be regarded as a point light source. For example, the sun you are familiar with is a point light source. On a sunny afternoon, under the direct sunlight, the projection of the object becomes very dark, with sharp edges and high contrast. At this moment, a white cloud came. After the sun is blocked by white clouds, it is still daytime around, but the projection of objects becomes soft. The relationship between white clouds and the sun can be regarded as a natural reference for reflective umbrellas and off-board flashlights.