That depends on the type and scale of the business in question. Businesses that specialize in launching different products may have their own in-house prototype team that can put together mock-ups and functional prototypes as needed.
However, we live in the outsourcing era these days, and it’s usually more cost-effective to have a third-party handle prototype production. At this point, who will take care of the job depends on what the prototype is made of. If the product in question is mostly metal, it may be outsourced to a company that handles sheet metal fabrication Birmingham, like the one found on this website.
Wood, plastic, and electronic fabrication companies will take on jobs related to those kinds of materials and processes. And for complex devices, the job may be outsourced to a specialized prototype fabrication company; these can usually handle a wide range of materials in the same place, although they may outsource the construction of large components.