The buyer owns the copyright by default ("unless clearly stated otherwise"), but only because the Fiverr Terms of Service say so
(except for work created through their Logo Maker, which has different rules)
I thought it would be seen like regular paid work, as in everything you create at work is property of the company employing you to work there.
This isn't quite right. As I explain in more detail here, because this is a contract job and not regular employment, absent a written and signed agreement stating otherwise, the work would be owned by the contractor.
However, their Terms of Service state (emphasis added):
When purchasing a Gig on Fiverr, unless clearly stated otherwise on the Seller's Gig page/description, when the work is delivered, and subject to payment, the Buyer is granted all intellectual property rights, including but not limited to, copyrights for the work delivered from the Seller, and the Seller waives any and all moral rights therein. The delivered work shall be considered work-for-hire under the U.S. Copyright Act. In the event the delivered work does not meet the requirements of work-for-hire or when US Copyright Act does not apply, the Seller expressly agrees to assign to Buyer the copyright in the delivered work. All transfer and assignment of intellectual property to Buyer shall be subject to full payment for the Gig and the delivery may not be used if payment is cancelled for any reason. For removal of doubt, in custom created work (such as art work, design work, report generation etc.), the delivered work shall be the exclusive property of Buyer, and Seller assigns all rights, title and interest in the delivered work.
This written contract accepted by both parties likely qualifies as "the parties expressly agree[ing] in a written instrument signed by them that the work shall be considered a work made for hire," which is the requirement to transfer copyright ownership to the buyer.
If a Fiverr-like platform did not have such a clause in their Terms of Service, the artist would own the copyright by default, absent a written and signed agreement stating otherwise.
Note: this answer describes how this works under United States law. The default ownership of contracted works may differ in other countries, though I expect that copyright-assignment clause would be sufficient in most countries.