Siri?” and a fourth inward-facing microphone to help isolate sound coming from the speaker to improve voice detection when music is playing.

Source: https://www.devicemag.com/using-homepod-for-conference-calls/

Their custom active noise canceling technology uses two microphones, an inward-facing mic and an outward-facing one to detect outside noise and generate an anti-noise waveform to cancel it.

Source: https://www.devicemag.com/how-to-reduce-background-noise-on-airpods-pro/