My impression is that it is often a series of random companies. A "factory" doing some part, or assembly, might not be larger than a class room. These days you can pretty much just search for something like "tablet factory shenzhen" and see how it is [0]. Of course then there will be larger ones like Strange Parts visits [1], and sketchier ones [2]. And then on top of that will be all kinds of companies doing design, distribution, sales and whatnot.

[0] e.g. https://www.youtube.com/watch?v=AfyAjkPIYyc [1] https://www.youtube.com/channel/UCO8DQrSp5yEP937qNqTooOw [2] e.g. https://www.youtube.com/watch?v=NZh7VfxBmcw