There are two important parts involved when turning on a PC: BIOS (or UEFI) and the operating system (OS).
They are not the same, and they do different jobs.
What BIOS (or UEFI) does
UEFI is the modern version of BIOS. Many people still use the word “BIOS”, but most systems today use UEFI.
This software is built into the motherboard, so it is already there before anything else. When you press the power button, it starts first. It checks that basic hardware is working, then prepares the system so it can run properly. After that, it looks for an operating system and loads it.
Once that step is done, control is passed on to the operating system.
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What the operating system (OS) does
The operating system, like Windows or Linux, is what you actually interact with.
It runs your apps, manages files, and handles memory and devices in the background. When you open a program, save a file, or connect something like a keyboard or USB drive, the operating system is handling all of that.
So while BIOS or UEFI starts the process, the operating system is what keeps everything running.
Does BIOS stop working after that?
Not completely, but it also does not stay in control.
Some low-level functions may still rely on firmware-related components, like fan control or power management. These are often set through BIOS or UEFI settings. However, they usually run on their own, or the operating system may manage them instead.
So it would not be accurate to say BIOS is actively controlling the computer after startup. BIOS is not the one controlling your apps, files, or daily use after startup.
Why they are separate
They are built for different roles.
BIOS or UEFI comes from hardware makers and focuses on starting the system. The operating system comes from software developers and focuses on running everything after that.
Keeping them separate helps the computer start properly and still gives you the flexibility to install or change the operating system later.
Simple way to remember
BIOS or UEFI gets the computer ready.
The operating system runs your programs, manages files, controls hardware, and handles everything you do on the computer.
Source: Reddit thread