When we are born, we have a very limited "vocabulary" and our ability to vocalize is very basic and primitive. As we learn to speak in the language of those we hear in our environment (mostly our parents), we learn to make new sounds - mostly through trial and error as we attempt to emulate or replicate the sounds we here others make. Eventually our brain learns what to do with our body to make certain sounds and starts writing the code.
Speech and singing for most untrained individuals, is something that seems rather automatic. In other words our brain tells us we want to make a certain sound, and without conscious thought, our brain has learned how to control all of the things our bodies must do to make that sound. We have to blow air across our vocal chords, stretch our vocal chords a certain distance, hold our mouth a certain way etc. It's a very complex series of adjustments and mechanics that are all put into play without much conscious thought (other than "I want to make this sound").
Not every person makes a particular sound in exactly the same way as others. Some may have learned to sing certain notes by placing much of the sound in the mask of the face to use as a resonator, while in others, the same note may emanate more from their throat. It probably depends on what worked first during the early trial and error process. But once learned, it occurs automatically without having to think about it.
What you experience as you sing is what your brain has learned to do with certain sounds. You have also probably discovered that you can shift the apparent source of any given note you sing, by consciously manipulating the mechanisms involved in producing the sound. In other words you might be able to shift a particular note from chest voice to head voice or from head voice to falsetto. Or shift the sound more to the front of your face or to the back of your throat.
You have learned through unconscious trial and error to place certain notes and sounds in certain areas within your vocal tract. But through experimentation, you might discover that by shifting where the sound appears to emanate, certain notes may sound clearer or more resonant or have richer overtones. Or they might be easier to sing and control by redirecting where the air travels after crossing the vocal chords on it's way out of your mouth.
Don't be afraid to experiment to see if some notes might sound better or be easier to sing if you alter their apparent point of travel (where you place most of the sound). I could say apparent point of origin but the point of origin for all vocalization is the lungs. Without air there is no sound (not even a whisper). And technically speaking of course, all sound "emanates" from the lungs.
But keep in mind that if you do discover a better way to place certain notes, it will take repetitive and intentional practice and conscious thought to override the automated process your brain and body have ingrained.
Undoubtedly, singing (as a hobby or profession) is something that we should "train" our bodies to do more effectively and efficiently. This training involves overriding the code that our brain wrote in our infancy to trigger the process of making certain sounds and creating particular pitches.
We are all born with unique physical parts that go into play to make sounds. We have all learned to make each sound in our own unique way. My personal advice to anyone who wishes to learn the "best" area of the vocal tract to place all the notes they are capable of singing, as well as the most efficient and safest techniques to use to get the most out of their voice - is to solicit the services of a qualified vocal coach or voice teacher.
