Yes, starter cable and Alternator B+ cable will always have 12volts, but no ground so no current can flow
On starter motor the Relay connects the 12v to ground to activate starter motor
On alternator the voltage regulator gets 12v key on and that turns on alternator
In order for current to flow there needs to be a Path to Ground
When you turn something on you are not giving it 12volts, you have to give it 12v AND a ground, so current(electrons) can flow THRU the devices components
i.e. disconnect the Negative terminal on the battery, the ground
Now see how many electrical things work, it will be 0 of course, even though they ALL still have access to 12volts from positive cable
So for anything electrical to work it needs to have a PATHWAY for current to flow, in this case from Positive to Negative on the battery, remove either and no current can flow
And actually current flows from Negative to Positive, lol, but we didn't have instruments capable of seeing that until the 1940's and the whole positive/negative thing was entrenched, and doesn't matter really