Or, the supplier that made the alternators for mopar also sold them to
Ford.

In products there are off the shelf parts, somewhat custom parts, and
fully custom parts.

The off the shelf parts are parts the supplier has designed and then
tries to sell to companies that make products they could be used in. For
instance, at my previous employer, I had a need for electrical connectors
that were fluid resistant and could take quite a bit of heat. A connector
company had a line of automotive connectors off the shelf that they sold
to the auto manufacturers that exceeded my requirements but would be
better than designing our own because they were done and automotive
volumes made the price more attractive.

The somewhat custom parts are sometimes unique to a particular
manufacturer, sometimes they become off the shelf parts. I've had
suppliers alter parts slightly to significantly for the needs of the
product I was developing. I believe one such part actually became an off
the shelf part for the supplier.

Custom parts are designed by the buying company's engineers for the most
part and the supplier creates the tooling and produces the part. All of
this is charged to the supplying company. Hence the first two are much
more attractive options when possible because there is no tooling cost
for the first, the second there might be, but often there isn't because
the supplier sees it as something they can add to their catalog.

So, I would bet this giant alternator is something that was just sold to
both ford and mopar by the same company.

Another funny story... My grandmother's tempo needed a new idle air
control valve. I don't feel like trying to get to the dealer's parts
counter when open so I pick one up at NAPA. I go to swap it out and find
that the replacement NAPA brand part is the real ford part with the ford
oval and part number stamps machined off.

Now obviously, ford isn't selling them to NAPA, NAPA is getting them the
same place Ford gets them.