How does Nikon D3400 take black-and-white photos with color background by itself?

There are two ways. I won't say the gag of "making a black-and-white fake background"

First, select "Optional Color" in the color mode of the camera. Optional colors mean that the camera only records some colors, and other colors are expressed in grayscale. This method is suitable for situations where the contrast between the theme color and the background color is large. For example, the theme of "the girl in red between green mountains and green waters".

Second, post-implementation. Using the layer principle of Photoshop, the portrait is dug out and the background coating is decolored. Or use the mask function to make the color layer display only the portrait part. This method is the most accurate and operable, and can achieve very subtle results.