I saw in this Wikipedia article that there are a lot of flats and sharps in the modal natural chords. And I was wondering if it's because they all refer to Ionian.
So in Aeolian the "bIII" chord is read "take the 3rd chord from Ionian, and flatten its root; then use this flattened root and make a major chord from it since the roman numerals are capitalized". Does that sound right?
Borrowing chords from a parallel mode then is just a matter of picking chords from this table (I saw this video on modal interchange). So the resulting non-diatonic chord progression, for example I bVII IV bII, is still always in reference to Ionian (?). Meaning, bVII in this progression is read "take the 7th chord from Ionian, flatten its root, use it to make a major chord".