Shenzhen’s advantage is density. Components, housings, packaging, testing labs and freight forwarders are close enough that a sourcing team can compare options quickly.
For overseas buyers, the supply chain is more useful as a process than as a factory tour: identify the product, confirm a supplier, coordinate packing, inspect if required, then arrange shipping.
A local trading and sourcing company does not replace the supplier. It coordinates the steps so the buyer can work in English, compare options and keep the order moving.