How are gadgets made?

(I hope the theme of gadgets is relevant to the community)

There was an idea to produce a gadget, to convert an idea into plastic and microchips. I will describe all the way that I see as a designer; I’ll stop where I faltered. I hope there is someone more knowledgeable in the tasks who can correct the direction of the search.

So there is an idea. Drawn and modeled. The gadget fell into the hands of a cadre draftsman, who, together with a process engineer, brought the model to the stage of manufacturing a prototype. On the trideprinter, the disc was dusty, the body was molded in the production, the fasteners and everything needed. And here - stop. Technology-stuffing gadget innovation is not (for example - the player), then you can order it (buy?), But where? and how to find the intended manufacturer (or at least where to find it)?
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Thanks in advance to informed comrades.