Hello, is the straight line shot by Nikon D750 not straight and concave?

The distortion is mainly caused by the lens.

The above picture is a typical pincushion distortion, which is usually caused by shooting with the telephoto end of the lens. For example, if you shoot with a 24- 120mm F/4G lens and a 120mm lens, the picture will have the above distortion. This is mainly caused by the large zoom design of the lens, especially the entry-level lens.

If it's serious commercial photography, you can't use this lens. Generally, a professional macro lens such as 105mm F/2.8 is needed, which can control the distortion to a few undetectable. If you don't have professional lens equipment, you can use professional software such as Adobe Camera RAW or Adobe Photoshop for post-correction.