Power to everything Nothing works

The negative battery terminal, I bought new last year, was rusted where the cable sits.
Yup, I'm a dummy.

I'm always chiming in to have good grounds, and add a ground hub next to the battery, blah blah blah.
I even made sure the test light had a good ground by clipping it to the battery and stretching it all over the car.

This car has zero accessories, lights, heater, wipers, so I just replaced the battery terminals and drove it all last year.

The thing I do not understand is why the solenoid wire on the starter, not connected to anything, would light up the test light when I flicked any switch or turned the key on?