Virtual Machines If you've ever used a video game emulator, you're pretty close to the experience of a virtual machine. VirtualBox or similar will provide a window on your Windows desktop in which you'll boot a completely separate OS. That guest OS has no idea it's not running on real hardware; VirtualBox pretends to be generic PC hardware and translates it to system commands to the host OS. The two OSes aren't aware of each other, and they don't share data or software. Because VirtualBox pretends to be a generic PC, the generic drivers built into the operating system should work; I wouldn't put it past Linux to have VirtualBox specific drivers built in. When you download a Linux ISO and plug it into VirtualBox, it will very likely "just work." Watch a Youtube video on the process to see what I mean.

"is the operation of a program in VirtualBox a fair test of the operation of the program in the actual OS?" Maybe. Generally speaking software is going to run better on a native OS (running on bare metal) than in a guest OS in VirtualBox, first because there's not a whole other OS running on the system hogging up resources, and second in the case of like CAD software or games, access to a graphics card is kinda tricky. So games probably run very poorly in VirtualBox compared to on real hardware. There may be cases where software runs better in a virtual environment but I feel that's rare.

Live Environments. A live environment is running on bare metal; You will shut the computer down and then boot the whole computer from an external drive, typically a USB thumb drive these days. When you boot a Linux live environment, Windows is not running at all; Linux is in direct control of the hardware, and it can mount and write to a Windows partition if present. It is very unlikely to hurt anything unless you tell it to, and if you're really paranoid you can physically disconnect the Windows drive.

The live environment is designed to let you see the system run on the computer to make sure it runs well, and to provide the environment from which the installer will run. There are some systems that are designed to be run from the live environment, like TAILS, but in general it's a temporary utility.

Dual booting* There are some disadvantages of dual booting; it eats up more space, there's two OSes to update and maintain, and moving files between the two isn't always fun since Windows and Linux use different file systems (NTFS vs EXT4, for example).

They will sometimes fight; Windows likes to take back the bootloader and prevent the menu from showing, booting straight into Windows; that's always fun to fix.

There's also the issue of the Real Time Clock. Windows likes to set the hardware clock to local time; Linux likes to set it to GMT, so one or the other will read incorrectly. Both OSes can be set to do it the other's way; you can set Windows to use GMT with a registry setting, xor set Linux to use local time via a timedatectl command in the terminal.

It doesn't take much more time to shut down and boot to the other OS; there is a menu for you to choose between them, which can add a few seconds.

As for why bother with Linux gaming...you want to get away from Windows, right? That's why people bother making Linux compatible games. I don't want to keep Microsoft around just to play games when Linux can do it just fine.