If you see philodendron hederaceum and philodendron cordatum for sale, you might think they’re different plants. They’re not, they’re the same.
They also go by the name of heart-shape philodendron. So if this is the case, why do we have differing names? Usually, it’s because there are people who know it by that name. One example is monstera borsigiana. That name no longer exists within the hobby, but it’s still used by old school growers who like to differentiate large form and small form Monstera Deliciosa.
Philodendron Hederaceum is the correct name.
Philodendron Cordatum is simply denoting the shape of the leaf. Cordate means heart-shaped. So it’s just another way sellers are labeling their heart-shape philodendron.
So if you’re a new philodendron collector, or just new to the plant trade in general, all three plants are the same.