Is there a certain rule between aperture value and shutter speed in SLR photography?

There is indeed a relationship, a law, only a law of change, and there is no accurate quantitative value.

And the premise is that iso, that is, sensitivity, is a fixed value.

For example, in the case of iso 100, the aperture is 5.6. If the shutter detects 125, then you reduce the aperture. For example, if you receive 1 1, the shutter will slow down, such as extending the exposure time and accurately exposing.

There is no absolutely correct value to refer to completely, but the relationship among aperture, shutter and sensitivity is equivalent to a seesaw, where one value is given and the other two are changing. For accurate exposure!