The most common problem I see is not echo, but software audio ducking that happens as a result of using onboard speakers and mic.

Some people have a hard time realizing that they're interrupting someone else because that other person's audio is getting ducked while the laptop prioritizes mic input over speaker output - with the intent to reduce echo.

Almost. The laptop of the person being interrupted is essentially muting its mic temporarily to avoid sending an echo of the interrupter. You could say it's prioritizing its speaker over its mic.

Basically, of all the ostensibly unmuted mics, only the one with the loudest human is truly unmuted.

It's closer to half-duplex than full-duplex. Full-duplex with no artifacts requires no echo cancellation which requires headphones.

What does the term "duck" mean in this context? I'm not sure what you mean.

“Ducking” refers to lowering volume so that other audio can play on top of it. When an announcer speaks over a song in the radio, or when Siri lowers your music so she can talk over it - that kind of thing.

Try talking while they are also talking. You'll see the problem.

It's easy to have conversations with friends on discord where 3-4 people are talking at once all with headphones. However this has never worked on a zoom or hangout with less techy family members or work colleagues using ext. speakers.

That may be part of it. On calls that I'm on people generally don't talk over each other.