In American Sign Language (ASL)
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The purpose of a friendly greeting is not to seek in-depth information. It is to acknowledge the existence of another.
"How are you?" when asked in passing is the equivalent of asking "Are you well?" -- (which is a "yes / no" type question).
The friendly greetings "How are you?" and/or "What's up?!" are exceptions to the regular WH-question "lower your eyebrows" rule.
If you lower your eyebrows on these phrases you will turn them into actual inquiries (instead of friendly greetings).
Here is a link to an example of the friendly greeting form of "How are you?"
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See: HOW
YouTube link to "How are you?" (as a greeting) https://youtu.be/pmL1k0jsLD4
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