[theamk]

It has to be both.

The plug manufacturer has to provide many, many keying options (say for a 2-pin plug, one might have variants 2A, 2B, etc... which explicitly cannot mate with each other)

An engineer designing the car has to ensure all plugs with same keying options are interchangeable (2A is for power supply; 2B for door switch; etc...)

If the plug manufacturer does not provide enough keying options, this will be pretty hard to during design time.

[themaninthedark]

You are generally not using just one plug style on a harness so most of the control is at the engineer level.

Unless you are a big player, what plugs you are using boils down to what receptacle the component that you want to attach to use.