Operating System (OS)
The OS (Windows, Linux, macOS, Android) is the middleman program between the "iron" and every other program — it manages processes, memory, files, devices and permissions. Why: every program of yours lives by the OS's rules — what folder it may write to, what rights it runs with, why it gets "Access denied" — the OS decides all of this. When: installing/removing software, at the "Run as administrator" prompt, setting PATH, killing a hung process. How: the kernel distributes CPU and RAM among processes, drivers "translate" device languages, the user-account system decides who has what rights. Where: your PC — Windows; the vast majority of servers — Linux; your phone — Android (also Linux underneath). Who: everyone — but for a backend developer, Linux fundamentals will become mandatory (your server will be Linux, Phase 13).
What this asks you
What is the primary function of an Operating System (OS) in relation to security mechanisms?
and 2 more questions