A lot of the really cheap china stuff is built from surplus stock which has been obtained free or heavily discounted or from "calling in favours"
Buying used SMT reels which are almost empty are a good example of a potential cheap parts source.
The reject bin from pick-n-place machines is also a potential source of components, they just need to be sorted and one thing china has is really cheap labour. Since China has such a huge manufacturing market the rejects/surplus market is quite large.
Part sourcing can also get a bit more dodgy.
There's a big recycling market, where useful ICs are removed from old PCBs, cleaned and resold cheap.
And there's production run rejects. Where the chip isn't quite good enough to sell but still works. These rejects are often taken 'unofficially' and resold through more dodgy channels.
Side story: You can get 5x 100W LEDS for $20 on aliexpress which is stupidly cheap, but if you test them you find most have bright spots, dead leds or unbalanced parallel strings. They're factory rejects but still normally ok if you run them at a lower current. And, when i ordered the 5 i was actually sent 10
that' s another clue that the seller is expecting high failure rates so is giving extra.
Also, there is a big difference between the price you might pay at digikey or from the manufacture vs what it actually costs to make the chip in the factory. China culture is quite different to western. There's a lot of negotiating on price and favours between people. eg, You might have a friend who works in a semiconductor factory and maybe able to buy low quantity stuff at cost, or close to it. In return you might help the friend with some other deal or something.