Generally speaking, the retaining/transfer of copyright varies from contract to contract. Contracts can be for exclusive (most traditional publishers are like this) or non-exclusive rights, which means other places can publish the same work but the creator gets a much smaller payday because of it.
Usually, the more you give up as a creator up front, the more money the publisher gives you, but the less control you retain over your work. In most cases the smarter thing to do is to sell the rights and take more money up front because statistically speaking your chances of losing out financially are very slim. Very few stories actually explode into popularity to get adaptations, and most stories don't even earn out their advance.
Viz Originals I'm pretty sure are actual paychecks for professional illustrators on deadlines, with company backing behind production and advertisement. Depending on the terms it's a very serious (and appealing) gig.