There are a few DIY DCC decoders (yes, people do make their own) and the sites for those will have full schematics. Not that these will exactly match any commercial decoders, but the concept is close.
The truck pickup wires generally first go to a set of diodes to rectify the DCC power into DC. This is also tapped off to the microcontroller to provide a source of the DCC signal. The diodes will be 4 relatively large rectangles, usually black, with two connections to each one.
Small black rectangles with numbers like 103 on them are resistors. 103 means 1000 ohms, or 1k. Other small rectangles of different color are usually capacitors. Small black elements with 3 connections are transistors, tracing these often finds them connected to function outputs. The biggest block with the most pins is the main microcontroller, near the motor lead connections where will be either 4 transistors or a smaller chip which internally contains 4 transistors in an H bridge configuration. If it’s a sound decoder, there will be resistors and capcitors between the main chip and speaker leads, possibly transistors or another chip of the amplifier is external to the main chip. If the functions are current limited (those decoders where you cna connect LEDs directly, without external resistors), you should be able to trace back the function connection and find a resistor.
–Randy