At what point in a US airport have I been "admitted" to the US?

When you are lawfully stamped and your status written in your passport. (Note that passports aren't always stamped by the officer if using an APC kiosk (which, at most airports, many VWP nationals can), but even then you've been lawfully admitted - you're admitted when the electronic admission record has been created). The term lawfully is very important.

To be clear, wherever you are in the country and however you crossed, you are or at least in theory supposed to be accorded basic and universal human rights. For example if hypothetically before crossing immigration/passport control you get murdered, the USA will prosecute the murderer to the fullest extent of the law.

When it comes to immigration benefits and law however, until you cross immigration/passport control (and are admitted lawfully) you have not been admitted into the USA. For this reason you have very limited legal standing with respect to immigration law and benefits.

See INA §101(a)(13)(A)

(13) 2/ (A) The terms "admission" and "admitted" mean, with respect to an alien, the lawful entry of the alien into the United States after inspection and authorization by an immigration officer.

Thus after you cross (and not just cross but cross lawfully) immigration/passport control, then you have all the immigration rights. To make it even more complex, note that that even fter you cross, if for example it is realized that the immigration officer admitted you by mistake, you are not lawfully admitted. For example if you had previously committed a crime of moral turpitude (which makes you ineligible for a visa) but had mistakenly been awarded a visa by a consular officer based on your lies, and you used that visa to enter the USA through passport control, your entry was void ab initio because you were inadmissible from the very beginning and hence according to the court you were not lawfully admitted.

At what point in passing through these stages have I technically been admitted to the US, and acquired the full legal protections I expect there?

Since 1976 you can be as much as a hundred miles from the border and run into a permanent checkpoint and kiss good bye to your legal protections. So I would not try to split hairs over which line in the fine floor mosaic of a US airport you need to walk over before you are in.

Your rights have absolutely nothing to do with your physical location at any given moment.

They are conferred on you due to your status (citizen, alien, etc. etc.).

People usually confuse this but that is only because your STATUS is not CONFIRMED (and therefore, you do not witness the accompanying expected treatment in its manifestation upon you,) until/unless you cross a certain point.

However, your rights are completely independent of that crossing.
Therefore pretty much everyone is wrong.

The difference is knowing the true legal basis, vs. just summarizing what you witness...big difference, albeit a bit abstract-seeming to some.

There is no physical border inside an airport. Some would consider the transit area as some kind of no man's land but it isn't at all. Indeed, the transit area is governed by the country's law.

While arriving, even if you haven't yet passed the passport controls, you are already in the destination country. The passport controls is just there to allow you to get outside of the transit area. But you are already inside the country and the country's law are applicable to you.

On the other side, once the plane doors are closed, the plane is considered as an extension of the company home country territory. But while they aren't yet closed, the police can come into it and arrest someone. This has already happen in several well known cases.