it is used to organize the programs and secure them

and run programs that need shells, like ION

MirageOS is a shell, like ion

it gives games several useful functions their authors would otherwise have to make themselves, like some sprite functions.

All mirageOS games written for that calculator are written in assembly, if I recall correctly only the 89 and 92 have a worthy c compilier avalible (ti-gcc or something)

look for how to program ion programs, the only difference I can remember for mirageos programs is a header containing the program icon [this is well documented on the mirageos website]

to code in assembly it is much easier to do it on a computer, and test in a virtual environment such as vti or ti's flash debugger

you can enter and compile assembly code on the calculator in one of two ways:
use an app [better idea]
enter the hex into the basic editor and compilie it with the AsmCompSomethingWhatver command [a pain, and not a good intro to assembly]