To get you started, think of the segment led displays as a dot matrix display. Your display routines are simply transmitting the buffer data to the segment pins / digit pins. Your user application can load up whatever segment information they want into the buffer and it will be displayed there.
Let's say that you are dealing with a 8-digit 7-segment display, common cathod (aka max7219 clone).
You need to figure out a flexible way to define the segment pins
#define SEG_A 2 //led's segment A connected to pin 2
#define SEG_B 5 //led's segment B connected to pin 5
#define SEG_DP 10 //led's segment DP connected to pin 10
//macros to turn on / off a pin, common cathod
#define SEG_ON(pin) pinWrite(pin, HIGH)
#define SEG_OFF(pin) pinWrite(pin, LOW)
Similarly, you can define the digital pins
#define DIG0 11 //digit0 on pin 11
#define DIG7 13 //digit7 on pin 13
//macros to turn on / off a digit pin, common cathod
#define DIG_ON(pin) pinWrite(pin, LOW)
#define DIG_OFF(pin) pinWrite(pin, HIGH)
Now, those routines are for common cathode displays. But by changing them quickly, you can reuse the same macro names in a common anode display, or to use a npn switch / pnp switch. The rest of your code remains unchanged.
Using pin names helps repurpose the code in different project, or to facilitate layout / wiring.