The BIOS and CMOS are often times thought to be the same thing, but they are not. They are two different components of a computer, but they do work together to make the computer function properly.
The BIOS is a computer chip on the motherboard. This chip contains a special program that helps the computer processor interact and control the other components in the computer. Without the BIOS, the processor would not know how to interact or interface with the computer components, and the computer would not be able to function.
The CMOS is also a computer chip on the motherboard, but more specifically, it is a RAM chip. This is a type of memory chip which stores information about the computer components, as well as various settings for those components and other variables. CMOS allows you to change date/time and startup boot order etc. When you access "BIOS" on startup, you're actually accessing the CMOS because as you stated the BIOS cannot be changed directly, only by performing BIOS updates.
The BIOS program on the BIOS chip reads information from the CMOS chip when the computer is starting up, during the boot up process.
In short, the BIOS allows the processor to control things such as HDD, CDROM etc. Whereas, the CMOS is the information which determines what these components do and can be modified by a user.
Hope this helps.