An open-source stethoscope that costs between $2.5 and $5 to produce

And one which is treated as a status symbol, at that. Part of the reason a good stethoscope costs more is because it looks nicer, not just because it works better.

Genuinely curious, what standards exactly, for a stethoscope?

To use it, you get the cuff pressure high enough that you stop hearing a heartbeat in the earpiece. Start releasing pressure slowly. As it comes down, take note of where on the dial you start hearing the heartbeat. That's systolic pressure. Keep listening, and take note of where you stop hearing the heartbeat. That's diastolic pressure.

Using one feels kind of magic.

Usually they call me back to the hallway where they check my weight, then have me sit in a chair and check my temperature, pulse ox and BP, with maybe only a minute sitting down before they do the BP check. My BP is usually in the "hypertension" range there.

But, if they come back to the exam room after I've been sitting in that quiet room for 5 or 10 minutes and check my BP , it's almost always in the "normal" BP range (same as what I see when I check it at home).

Doctor calls it "white coat hypertension", I call it "rushed BP check in the hallway".

[1] https://en.wikipedia.org/wiki/White_coat_hypertension