It's a gray market. Sometimes, I mean sometimes, these chips are real and they usually come from a "leak" in the supply chain - unauthorized resales by small vendors, or "unwanted" surplus from a previous order or production run. Usually they exist only in small quantities and are stored and handled in unknown conditions, it's not something you want to use in production. But it explains why the AliExpress vendors always have some components when major distributors are out of stock (e.g. You can do it too - just order all the remaining stock of a rare chip from Digikey and put them on eBay...)

For example, I heard that a common practice in Shenzhen is selling reels of genuine components, but the "lot number" (serial number) on the labels are removed, so the manufacturers cannot trace these unauthorized resales back to the source. Another example, in a personal hobby project I needed to use a specialized controller that is not available on the public market, but a AliExpress vendor has them, it's likely surplus from somewhere.

> I can see those chips available in their thousands on aliexpress and similar sites for 10x the price

The quantity is not always real, sometimes they just put a random number here. And even when it's accurate, the sales occur via various online and offline channels simultaneously.

---

Disclaimer: It's extremely risky to order components from an unknown source, I do not endorse doing it. And my hypothesis on their origins is my speculation, so take a grain of salt.