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.
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.
[1] https://www.crowdsupply.com/machinery-enchantress/the-new-es...
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 :-)