Any generic guides for building firmware for devices with modded flash memory?

Are there any guides on what changes to make on the software side to get OpenWRT to recognize extra flash storage? Is it required to make changes to the bootloader? How should the partitions be rearanged to account for the extra space (if at all)?

Most answers to those questions depend on which router you are talking about.

Is it because of different flash types and layouts? Can it be generalized on those types or is it literally different for each device?

There is nothing generic, you are the (partial-) manufacturer (by modifying flash and other things), only you can know what you have done.