In uk.d-i-y Chris in Makati wrote:
Phone manufacturers source their batteries from various third-party
suppliers. Each of those suppliers probably sell their batteries via
different channels to any number of other customers.

So is a battery still "genuine" if it came from the same factory, but
didn't have a particular label on it?

I think the 'made in the same factory as Kelloggs' argument is overdone.
Most top-tier brands (Apple, Samsung, etc) have agreements with major
contract manufacturers (Foxconn, etc). Foxconn have enough of a reputation
to protect that they won't build knockoff iPhones on the late shift and let
them into the market.

For lower level components it's possible the same part is branded multiple
ways (like different cars can share the same part), but I imagine the volume
of Apple, Samsung, etc is such they buy all the production of factory X with
no scope for running a late shift. Plus if Apple found out the contract
would be terminated very fast. Most of the parts you might replace (screen,
case, battery, etc) are bespoke so there's no sharing between different
brands than there might be for oil filters or wheel bearings, where you
might buy a Skoda part cheaper than an Audi part.

So when we're looking at aftermarket parts I think it's fair to assume we're
looking at those from secondary suppliers. The QC is only as good as the
effort they put in and the supply chain that got it to you.

This is separate from the increased risks of fakes in 'genuine' parts, as
against aftermarket brands which aren't worth faking.

Theo