The devil is in the details. Different contracts state different usages.
Often, I’m hired to make things for folk, and they own it entirely. I see these things out in the world, I sometimes see other artists hired to butcher it to fit a new purpose. But that’s OK, I account for that, and often I hand over the source files from the things I make… Layered documents etc.
However, there’s a really disturbing trend of large companies appropriating fan art and claiming that because they own the IP any derivatives belong to them too. This is far ickier.
The main thing though is credit. You’d think that giving a nod to the original artist would be nice. It costs nothing and can have a massive impact on their business.