ECU operation

Why do some ECUs have an internal ground, external ground, or both? I know they need a power and ground, receive inputs, and send output, but is there anything I’m missing? Can we get into detail of why they’re wired the way they are and how they operate?

Usually there is some filtering or isolation between the various grounds. For accuracy at high-precision inputs, you need a to know exactly what reference ground for that input is. Noise on an external ground may exceed the resolution of the input, and thus your measured signal would have noise on it. By connecting that inputs's reference to a specific internal ground, you may be able to eliminate that noise and improve the signal fidelity.