For car/truck ECU's the IC's are custom. It depends on the era, what year.
The MCU/TPU are usually slightly modified stock items. More memory, faster clock, added timers, added I/O ports kind of thing. If you rummage around the firmware you can figure it out.
Analog and power IC's are ASICs. Again, some are off the shelf with modifications.
GM's volumes allowed them to get Motorola, SGS etc. to make tailored parts.