How to automotive/boat devices limit voltage to 12 volts?
Devices designed for automotive environments have a wide input range that covers the typical automotive 12V rail. They spec the input at 15V on average. Most cars have a nominal voltage of 14.6V while on and charging via the alt.

They work fine at these raw fluctuating voltages. Some like motors or fans do not need regulation. Others, like radios or computers, require cleaner signals so they internally regulate down/up and filter the noisy 12V rail as best they can.

If you need a regulated 12V input, use a low dropout voltage regulator, or a switching regulator.