There's 'sourcing agents' whose job is to coordinate the garages. They don't work for any of the garages but as a westerner you'll get in touch with the "electric motor guy" who knows all the factories to contact for that particular purpose. They'll meet you at the airport gate and you essentially pay them as a guide to negotiate the shanghai business environment.
Yay and nay. That's only for very small manufacturing stuff. To assure the quality control and lower the price, it will eventually head to the large scale factory. The difference between what happened before and now is that, the minimum order quantity has gotten so low (thanks to CNC and computerization etc), now bigger factories can even handle MOQ down to 100.
I would advise you against going to those smaller factories -- QC is a nightmakre. Problems will arise. When you go to Canton fair or Yiwu for trade shows, I always, always, always recommend you to make a factory visit, and for the first batch, have a reliable Chinese person you trust to fly there and do the QC (if you hire someone that you barely know for QC, the other side might just bribe him off) and you will end up getting garbage when it gets to Long Beach port.
That's what Naomi Wu really did while producing influencer type maker videos.
"She knows Shenzhen like the back of her hand." Someone who bought her services describes her leading them confidently into sketchy alleys and up back stairs to unmarked doors, behind which were people making the thing they wanted.
She has a book about how to get electronics done in Shenzhen.[1] There was a previous edition published by a westerner, but Wu, a Shenzhen native, updated it.
In ten years, Shenzhen changed a lot.
It's not really so much a book on how to get electronics done, more sort of an electronics specific phrase book, full of the names of things you can point at when looking for stuff in the markets.
The original was written by Bunnie Huang (a name to be sneezed at in his own right) who (I assume) grew up speaking Mandarin at home. Naomi is a local though and probably has broader local knowledge.
The original Shenzhen markets are still there (I've been going for a decade or so, they haven't changed that much), they're getting a bit crowded out by consumer electronics - I can still go and buy great tools in HQB at same places I was 10 years ago, reels of resistors at the same places, people will still sell you cut tape from part reels on the street - the big change are the electric bikes, you take your life in your hand each time you turn around :-)
Probably the first factory you walk into? That one would become the prime contractor that manage suppliers and subcontractors - I think those are the terms they use in multimillion aerospace and defense programs, but I believe the concepts are the same.
For aerospace programs, you sign a contract with e.g. Boeing for a 787 to be delivered 5 years for $500m, to be delivered to your company pilot at the Boeing Everett facility to be signed off and flown to accepting aircraft maintenance facility for your fleet. They then start dispatching fuselages and wings and parts to various aerospace kilo-corporates. They occasionally call you for confirmations and approvals for important items like engines and small overruns.
I guess the same in Shenzhen happens as a mostly-verbal order for a motor against Shenzhen Suchandsuch LLC in 3 days for $15k delivered the back of the factory on a shop table, with their chief machinist there to get you through the artifact and to help you with bubble wraps and duct tapes. But the overall structure is the same.