First, its not "he". He and she refer to male and female living organisms, usually higher level mammals like humans. The proper pronoun is "it". The BIOS is a "thing", not a person.
A little English lesson never hurts, otherwise, you won't get better at it
And you will sound like Borat
In any event, BIOS is software, its the startup software for a computer or embedded system. When the computer is powered it, its nothing more than a microprocessor connected to external memory (RAM and ROM). Thus, you need to have bootstrap or start up code that gets the computer going and has "BASIC" input and output functionality. This is what BIOS does. On early PCs BIOS was much more important since operating systems like DOS actually used the code in BIOS after the start up of the computer. Now, BIOS simply boots the computer and then loads in the command or OS kernal which takes it from there. And in many case, modern OS's do NOT use any functions from BIOS.
So, the bottom line is when a computer boots, the processor goes to a SPECIFIC location in memory and starts executing code, this is where the entry point of the startup system/boot system is. Then early PCs BIOS stored a library or API of functions to help OS authors communicate to the hardware via a hardware abstraction layer. But, these days this is rarely needed, but is still there for legacy and backward compatibility.
Therefore, to answer your question -- BIOS is software, it is NOT connected to anything. The computer is the computer, it has a microprocessor, buses, and interfaces, then the startup code runs which has a library of functions burnt into ROM called BIOS, this software KNOWS about the hardware and thus can command the hardware, but its not connected in any way. Hardware is accessed via hardware interfaces, memory mapped IO, or IO mapped channels, ANY code can access it; BIOS, the OS, a game, whatever -- its just that years ago, when you wanted to write a byte to the CD, there weren't device driver models, there was a BIOS driver model, and thus you would call an interrupt or a BIOS service by vectoring your code (making a jump or calling and ISR) and then this would call the BIOS API for writing a byte, or whatever. And how you talked to the API was thru register and or stack space based on an agreed set of variable and data structure conventions.