This question comes out of two recent questions and answers about modal music:
@Athanasius gave very helpful answers, especially explaining that modes aren't fixed scales, but more like melodic conventions or archetypes, and especially important with flexible treatment of certain degrees of the (scale/mode/key.)
The explanation in What are modes in the real world? by comparison to minor mode conventions - like the treatment of the leading tone - made clear how "mode" is more about melodic processes.
That most obviously translates to being in Dorian mode, where the third is minor and the sixth is often minor and the seventh is raised for a leading tone. Basically, old Dorian mode is modern minor key. Aeolian wasn't a name used historically and to the extent that someone would use the name - or natural minor - in modern times the "modal" handling is still essentially the same as Dorian or modern minor key.
Phrygian is unique compared to the other minor modes. It has a minor third and sixth and its leading tone is the half step above the tonic. Seemingly it doesn't need adjustments of tones like Dorian/minor mode. In terms of melodic process being in Phrygian mode would seem to mean not needing to raise/lower tones. It's more purely diatonic.
If that description of minor mode is more or less accurate (I hope it is,) my question is: what are the features of being in the major modes?
From the earlier questions I understand that Lydian was the historic name and Ionian was never used. Lydian mode frequently lowered the fourth degree from an augmented fourth to a perfect fourth above the tonic. In doing so Lydian and so-called Ionian become roughly the same as modern major key.
Is that the primary modal aspect of Lydian?
What happens with Mixolydian beside raising the seventh for a leading tone?
In terms of modern major keys, a raised fourth and lowered seventh are used to tonicize the dominant and subdominant respectively. It's noticeable those are the two variable degrees in Lydian and Mixolydian. Can the two somehow combine to a major mode similar to how minor mode has its own set of variable degrees? My first thought it "no", because #^4 and b^7 change the tonic, whereas variable ^6 and ^7 in minor don't change the tonic. But that may be a hasty conclusion from a largely modern, harmonic perspective.
I fear I'm just repeating the earlier discussions in the two old questions. Are Lydian, so-called Ionian, and Mixolydian all really reduced to one major mode?
If the true difference in those modes lies way back in early Chant, and the distinction truly was lost by the times of early polyphony, that basic knowledge is helpful too.