I have tried Debian on VirtualBox and would like to check out Raspbian. How do I install Raspbian on VirtualBox?
VirtualBox lets you run x86 virtual machines on an x86 processor. Raspbian is a distribution for ARM processors. Raspbian cannot run in VirtualBox.
Raspbian is essentially Debian with binaries compiled to match the Pi's processor more closely than the official Debian binaries. So if you're going to your system in a virtual machine rather than on Pi hardware, there is very little reason to run Raspbian and not Debian.
If you really want to run Raspbian in a virtual machine on your PC (again, for most purposes, you might as well run Debian x86), you need one that emulates the hardware, such as QEmu. See Emulation on a Windows PC
Raspberry Pi Desktop is also available. You can download that and run it in a virtual machine. Just go to the following link and scroll down to Raspberry Pi Desktop (for PC and Mac)
Just to clarify, the Raspberry Pi Desktop is compiled for x86 processors but is otherwise the same Raspberry Pi OS that you are used to. You can run it on a physical or a virtual machine.