Fake TL072/TL074 IC's - how does the economics of it work?
The factory producing the wafer of the IC isn't necessarily the same factory as the one cutting the wafer into individual chips and packaging them in plastic. Packaging can be bought as a service!
As you can imagine, while producing modern¹ wafers is high-tech, there's decades worth of relatively low-tech packaging systems by now, sitting in factories not owned by the high-tech producers.
So, there's enough people with access to cheap machines able to label IC packages. And, as in every industrial setting, there's some trash can where a couple thousand of damaged / unknown state / overproduced / nightly production where the supervisor didn't look very close / … ICs end up that you can relabel.
I mean, I pay very little money (€7 for 100 TL074CDR for instance)
Um, Arrow sells >=100 pcs for 0.10 € a piece, so that's not that much of saving. My arrow orders basically always arrived the next day, so I don't think you're doing yourself a favor, even if you happened to buy original parts.
By the way, the time-proven TL074 is certainly not the cheapest opamp from its class on the market; arrow, for example, sells single-channel opamps for 3 ct (!) a piece, which is more cents per channel, but I've pretty often used a quad-channel opamp where I only needed 3 channels (or fewer!).
¹ The TL074, from 1979, is not a modern chip; the factories that originally produced it have long been sold (low margins in that sector of low-tech chips!) to companies who are willing to compete on the least-end market, or do custom chip production, or only run for really large runs of cheap ICs.
There isn't a way to spot them until you test them, and the most reliable way is going through proper distribution channels to source parts. This means sticking with major distributors like Mouser, Digikey, Farnell, etc. They buy parts directly from the factory and check their sources. Buying on AliExpress has no protection, and you could be buying anything.
It probably doesn't cost much to relabel IC's. I heard that most counterfeiters don't even try to match the functionality and only match the package. Once they've made the sell, it doesn't matter to them; the money has been made. If you could take 1$ worth of parts and resell it for 6$, you've made 5$ and for some people in China it's worth it.
Source: Is That Part A Counterfeit? Here Are A Few Pointers