Careful using the term 'fake'.
A fake is something that claims to be what it is not. Such as a cheap copy with packaging to appear an original equipment part.
An aftermarket branded part is not a fake, merely an alternative, and they range from better than OE through to 'paperweight' quality.
There are not that many thermostat manufacturers, so many aftermarket brands may be identical, and some may be from the same source as OE. Always best to go with a trusted brand name.
Even OE can change source, often to a cheaper source once a part is no longer used on new car production. Many component manufacturers can only supply at economic prices when quantities are large. Once new cars no longer use a component, the replacement parts are sourced elsewhere. So OE, may not be the same as originally fitted.