your mixed voice as in voce mista is primarily taught as a airy texture introduced into your voice; in full voice, formant develops when the physical acoustics of your resonant spaces coincide and create distinct frequency zones in and around your head that naturally amplify laryngeal modulations.
depending on voice type, your mixed voice will most likely be obvious in rounder, concentrated "oo" vowel shapes, straight tone with no vibrato. Experienced singers in jazz scat will know how to blend real with mixed voices, creating the illusion of ridiculously high voices.
I'm a jazz-scat singer with a comprehensive range from low B (2nd ledger below bass clef) up till high Db (above treble clef -8ve) in real voicing, and something up to a soprano G in mixed voice. so that's B2 - G5 on a normal day.