How does Erham Alman add harmonic richness to his guitar tone?

The Behringer UC200 Ultra Chorus is the key pedal here: it adds depth and modulation to Alman's melodic passages, complementing the high-gain distortion in his rock and metal sound.

Strictly speaking, chorus creates a sense of thickness and movement rather than generating harmonics in the way distortion does. That combination of atmospheric modulation and gain helps Alman balance crushing riffs with intricate melodies.