They need each other: 'singing position' and 'placement.' How to find each aspect? It's not as hard as you might think, that is, if you have an ear for it.
The classical singer can find his/her singing position by speaking a clear, deep and resonant vowel. I call this CDR for short. What happens when you can make a clear, deep and resonant vowel on a lower (approximated) pitch with ease and not too much volume? (Not making a lot of volume makes you listen.) Larry the Larynx goes down a bit without any fuss. He just goes there, often without the student even being aware of it. Once a clear, deep and resonant vowel is obtained, the teacher can then ask the student to observe what happens when the spoken vowel is not clear, deep and resonant. "Oh!" They often say. "It doesn't go down!" Meaning Larry. Of course, the danger comes when the student gets fussy and tries to keep Larry on the Low instead of listening to the clear, deep and resonant vowel. Funny how that is. Students can be prone to controlling the voice mechanically if they are given the chance, but my observation is that- without exception - this always bites them in the butt. To get them to stop doing that? This means listening to a clear, deep and resonant vowel.
Are you aware that I've written CDR five times now? Get the idea? The teacher has to keep this in the student's consciousness until they are able to obtain it on all vowels, closed or open. What does the student hear when they are doing this? Larry vibrating with tone. My own teacher called this the 'core.'
Once CDR has been obtained on all the vowels in the speaking voice, the student transitions to singing without letting go of the singing position. However, the singing voice is different from the speaking voice in one crucial aspect: instead of the attention being on the 'core,' it must shift to the level of the face, head and eyes. This takes some practice. Of course, five minutes of demonstration reveals a great deal more than the written word. It has to be heard. And what is heard? The old term is placement. A more modern term - or explanation - is bone conduction. One hears the head ringing with tone. Paradoxically, it also seems to be outside the head. This is the air-conducted aspect.
The higher one goes in the scale, the greater the sense of placement, the stereo-like sense of the vowel swimming around the head and face. It doesn't appear, however, if the singing position has been lost. In this sense, the singing position and placement aid one another: higher notes beget placement, while lower tones singing position. It's simple, elegant and - dare I say - easy, if you can hear what it means.