Manual-enabled modes give the photographer control over the various parameters of exposure. There are three exposure parameters – aperture, time (shutter speed), and sensitivity (ISO), and in different modes, these are each set automatically or manually; this gives 23 = 8 possible modes. For a given exposure, this is an underdetermined system, as there are three inputs but only one output. Accordingly, there are many combinations that result in the same exposure – for example, decreasing the aperture by one stop but increasing the exposure time or sensitivity to compensate, and there are various possible algorithms to automatically choose between these. source: https://en.wikipedia.org/wiki/Digital_camera_modes
https://www.cambridgeincolour.com/tutorials/camera-exposure.htm
https://photographylife.com/what-is-exposure