What do you think “fake” means in this context? That the part is really a Loony Toons sketch of the real thing, stickytaped to the surrounding engine? No, it means it’s essentially an outwardly identical part without the appropriate QA/QC processes undertaken to certify it as genuine and to spec.
This may for example include destructive and nondestructive testing of parts, or it might include tracking the manufacturing process parameters to ensure correct outputs. For example, a cast component has sufficiently low levels of porosity, inclusions, etc. that might in time lead to premature failure. Or it might mean that there wasn’t sufficient record of the brew of the metal going into the casting or the casting process itself to confirm the part has the requisite material properties. In all but the most dire circumstances, it’s nearly impossible to verify whether any of that was done correctly just by looking at the part; for all intents and purposes, it’ll look identical to a real part.
In the case that the OP references regarding Virgin Australia [1], the parts were a low-presssure turbine blade and a high-pressure turbine nozzle seal. Both of them would fall into situations similar to the above. If cast, the turbine blade would need to prove what I outlined, amongst much more. If machined from billet, it’ll still need proof the ingot itself was manufactured correctly which mirrors a lot of the above. In the case of the seal, there’s similar processes to prove that the non-metallic material is to spec so that a Challenger-style failure doesn’t occur, except within expected operating parameters.
The link I provided regarding Virgin Australia gives much more detail about the core issue at play. This isn’t a situation where manufacturer B is stamping its parts as made by manufacturer A. It’s where manufacturer B says it can make a part compliant with the specifications, same as manufacturer A, but then does not have the required documentary evidence that it meets those specifications.
1: https://www.smh.com.au/business/companies/two-virgin-aircraf...