A question about is there a sensor that shuts down the alternator behind the dash

There is no sensor that shuts down the alternator. A voltage regulator integral to the alternator controls the voltage.

So it's possible the voltage regulator needs to be replaced or does the alternator need to be replaced

The voltage regulator in integral to (part of) the alternator. Get it tested either on or off of the vehicle before buying a replacement as a wiring issue can also cause charging issues.