This short clip from Ableton's Learn Live series walks through two specific parameters in the Chorus-Ensemble device: Ensemble mode's third voice and the width control shared across modes.
Switching to Ensemble mode adds a third evenly-split copy of the signal alongside the original, which is what gives the effect that denser, multi-person quality. Bringing up the amount and rate pushes it toward a full room-of-voices sound, though a little restraint goes a long way.
The width parameter controls how those individual copies are spread across the stereo field. At zero, you get a mono chorus. Pushed to 200%, the copies fan out to an extreme spatial spread. It's a simple range, but it gives you real control over whether the effect sits inside the mix or opens it wide.