Regarding the OP, it depends on what you're buying. For instance, if you were to buy a Nordic nRF52L01+ on Aliexpress, you will most likely receive a counterfeit. It will look like a Nordic and have the Nordic markings, but it won't be a Nordic, and it may not play the same. Likewise, if you were to buy a branded CR2032 coincell battery on Aliexpress, you will most likely receive a fake that won't last as long as the real thing.
On the other hand, if you want to buy atmega328p chips at $1 apiece on Aliexpress, it's probably the real deal and a really good price to boot. At least that's been my experience.
Almost all supercapacitors sold on Aliexpress are total junk. They aren't counterfeits. They're simply garbage that don't perform anywhere near the advertised specs. Oh, and datasheets? You probably won't find any.