I'm an occasional iPXE contributor, I guess it's my time to shine!
Bootloaders are reading block of data from CR-ROM, disk, floppy or USB drive by using int 13h[1] whose handler is set by the BIOS. A bootloader is only asking the BIOS to read data for him and does not have to implement drivers.
iPXE replaces the int 13h handler in the interrupt vector table[2] by its own, and when a read is issued by a bootloader, it performs it with an HTTP range or iSCSI request.
It's actually really simple. :)