A prototype part is a purchasing exercise. A prototype build is a project, because it has an enclosure, a mechanism, fasteners, maybe a board and a cable loom, and somebody has to own the fact that all of it fits together. Choosing between suppliers is mostly a question of how much of that ownership you want to keep and how much you want to buy.
Three shapes of supplier
A parts supplier makes what your drawings say and nothing more. A prototype shop makes the parts and assembles them, and will usually flag a clash before it builds it. A development partner takes the requirement and returns a working unit, doing the engineering as well. Prices differ by an order of magnitude across those three and so does what you are accountable for.
The questions that separate them
Who owns the design files at the end. Who is responsible if two parts do not fit. Whether finishing, fastening and assembly are included or quoted separately. Whether they will build a second unit from the same files without redoing setup. And what happens when a change arrives mid build, because that is the moment the relationship is actually tested.
Planning the rounds
Most builds want a fit round in the cheapest process available, a function round in materials that behave, and an appearance round with real finishes. Trying to get all three from one build is the classic overspend, because it forces the cosmetic finish and the production material onto parts whose geometry is still moving.
What to hand over
A parts list with materials and finishes, models and drawings, an assembly drawing or a short exploded view, and a note about what the build has to demonstrate. That last sentence changes how a good supplier builds it: a unit for a photograph and a unit for a drop test are assembled differently even from identical parts.
Questions people ask about prototype manufacturing
What is prototype manufacturing?
Making the parts and, usually, assembling them into a working or representative unit before production tooling exists. It spans anything from a handful of machined parts to a full functional build.
Should one supplier make everything?
It removes handovers and the arguments about whose part is out of tolerance, and it usually costs a little more. For a build with a mechanism it is generally worth it.
How many prototype rounds should I budget for?
Two or three for a moderately complex build. If you are past four, the rounds are usually testing things nobody wrote down at the start.
Do prototype suppliers keep my design confidential?
Reputable ones will sign a confidentiality agreement before quoting. Ask for it up front rather than after files have been sent.