Tuning in a a cappella choir is not fixed by a keyboard. The singers adjust to each other, constantly, by ear.
"You aim to disappear into the chord, not sit on top of it."
"We stagger our breathing so the line never stops."
"A pure third is slightly lower than a piano's. You feel it lock."
"If the words are clear, the music mostly takes care of itself."
About the ensemble on the about page.