the application of aperture has two main functions. First, adjust the amount of light entering. Under the same conditions (referring to the same exposure time), the amount of light entering will be large if the aperture is large (f value is small), and vice versa. When the ambient light is insufficient, and the shutter speed has been increased to the limit (it will be unstable if it is higher, also called safety shutter), it is necessary to use a large aperture to increase the amount of light entering.
second, it is used to adjust the depth of field. The larger the aperture, the deeper the scene, and vice versa. When it is necessary to highlight the subject and blur the background or foreground, use a large aperture (small f value).
after understanding this principle, you can choose the aperture according to your own needs. If you want to blur the background, you need to use a large aperture (some introductory lenses or card machines can't achieve the purpose of blurring because the maximum aperture is F3.5 and the focal length is not long enough). When the light is insufficient, use a large aperture. Now I can only tell you the basic principle, which should be coordinated with the focal length and exposure time. You need to experiment to master it.