Modes should be considered ‘numerically’, 1,2,3, etc.
Not, alphabetically, C,D,E.
Modes are both, ‘Serial & ‘Paralell…
( meaning there is… C Ionian, D Dorian, E Phrygian, etc.
and, C Ionian, D Ionian, E Ionian, etc. ).
The ‘Key Tonal Center’ can be any Tone-Note, of your choice…
However: that 'Tonal Center is ‘Ionian’, or ‘number one, ( #1), of the Modal Sequence.
(example ) … find the ‘Key Note’ Bb, this is now #1- Ionian,
C, is Dorian, D, is Phrygian, E, is Lydian, etc.,.
Think of #1 as the ‘Major Center’ / Not, just, C’ .