It is called polyphonic singing aka overtone chanting, harmonic singing, or throat singing. Bjork does some (but NOT all) of her singing in a polyphonic manner.
Also refer to Wikipedia documentation as follows:
Overtone singing – also known as overtone chanting, harmonic singing, or throat singing – is a type of singing in which the singer manipulates the resonances (or formants) created as air travels from the lungs, past the vocal folds, and out of the lips to produce a melody.
The harmonics (fundamental and overtones) of a sound wave made by the human voice can be selectively amplified by changing the shape of the resonant cavities of the mouth, larynx, and pharynx.[1] This resonant tuning allows singers to create apparently more than one pitch at the same time (the fundamental and a selected overtone), while actually generating only a single fundamental frequency with their vocal folds.
Each note is like a rainbow of sound. When you shoot a light beam through a prism, you get a rainbow. You think of a rainbow of sounds when you sing one note. If you can use your throat as a prism, you can expose the rainbow – through positioning the throat in a certain physical way, which will reveal the harmonic series note by note.[2]
https://en.wikipedia.org/wiki/Overtone_singing#Mongolia_and_Buryatia