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This is achieved using Acoustic Echo Canellation (AEC). This essentially subtracts the output of the speaker as well as reverberations from the room from the microphone input. Here's a youtube video explaining the basic principle: https://www.youtube.com/watch?v=bJKGrheOoY4

Source: worked on 3rd party Alexa speakers

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Fundamental principle at work is adaptive filtering.

it also has uses in noise canceling headphones, voice conferencing software, and radar/sonar in some cases.

No LLMs or deep learning at all - purely DSP!

This is also the reason you can (sometimes) video conference on your laptop without headphones plugged in. The software does not (or at least tries not to) record its own output.

You only notice it if you try to use different devices for speakers and microphone - you'll get a feedback loop, since the AEC only works on the same device.

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