BAT54c and TVS SM6T39A application usage?
The battery of a car may be nominally 12V but the system produces lots of spike (transients) voltages far bigger than 12V. 

The SM6T39A (39V) is a transient suppressor and limits the potential damage this spike could do. 

The BAT diodes have one end tied to 3V3 and act as a clamp circuit. Any voltage in excess of about 3V5 (3V3 + forward diode drop) will be 'clamped' at this level so further protecting the (sensitive) inputs to the microcontroller.