From the S-1: "We use DRAM ICs produced by Samsung, Micron and Hynix in our DRAM modules. We purchase those DRAM ICs, pursuant to purchase orders and not long-term supply contracts, largely from third-party distributors and, to a lesser extent, directly from those manufacturers."
Edit: And "We maintain limited manufacturing facilities that only produce DRAM modules, custom built PCs, custom cooling and performance controllers, and as a result, we depend entirely upon third parties to manufacture and supply the gear we sell and the components used in our gear such as gaming peripherals and gaming components." So they do actually manufacture the modules in-house.
Micron, Samsung, Hynix like most others. They aren't reselling ram, just using chips from the only companies that make them, but they pick certain bins and do their own QC and direct support etc.
Besides Crucial/Micron, it seems like consumers can't buy DIMMs from Samsung or Hynix; you have to get them from some gamer brand. And clearly a lot of people are buying from brands that they recognize.
What makes you say this? I always buy Samsung DIMMs. They are always in stock at Newegg and on the shelf at Central Computer (SF Bay Area brick and mortar computer store).
You can buy DIMMs from all three just rare to see in most retail channels for consumer stuff. Obviously Micron have crucial, but you see Micron non-server DIMMs in OEM systems.
DRAM makers sell their chips only in JEDEC specifications, everything faster than that (3200+) needs to be binned by the module maker. For example, 3600CL16 is the standard choice for AMD systems, 3200CL14 is also very common and has much more aggressive timings than JEDEC.