most all operating systems and some addons like java work on API - aplication programmers interface.
all it is is a bunch of subroutines designed to make life easier. simular to bios only held in memory of the computer.
if you need more detail look here
https://en.wikipedia.org/wiki/Application_programming_interface
if you find a good reference book please post it.
all it is is object code routines. easily disassembled to be modifed or whatever. including copied out of it and added to your code so you do not have to pay license fees for using them. something bill gates use to do, do not know if he does anymore,
basically all .DLL are the same as this. callable object code routines.
DOT NET is the same. starting from dos to end up as it is. I do not trust it much as anyone who can create a program to do this obviously also has the reverse engineering decompiler as well. personally i hold my own code sacred not to be copied or stolen. that's why i do not use many of these and rely mostly on bios calls or ripping out the routines i need. modifying them to what i need and adding them to the code itself or a seperate .DLL library. Same can be done for bios routines so you never have to touch bios calls