For a general introduction, see the Audio Feedback article in Wikipedia.

For our purposes in the PACE classroom with Zoom:
Phone cameras being used to show the classroom to people remote on Zoom
should be muted in Zoom, and should have all audio (mic AND speaker) minimized or silenced.

At some point in the future, it might be feasible to use the mic on a cell phone.

But care should be taken to keep Zoom cell phones away from other microphones
or speakers in the room. Feedback is almost always caused by cell phones,
seldom by any of the audio equipment installed in the room or on the cart.

Feedback can occur in the classroom if the amplifier settings in the audio cabinet are
set too high. The top of any slider should be no higher than the bottom of the yellow tape.