Century Micro, a Japanese maker of memory modules, has published images of its DRAM IC test fixture that is used to identify defective memory chips on RAM modules (via HardwareLuxx). Such fixtures are available from various retailers and can be used to repair failed DIMMs or even build DRAM modules with cherry-picked memory ICs to hit overclocking records.
When leading makers of high-performance memory modules for overclockers say that they use hand-picked ICs to build such DIMMs, they are not entirely accurate. They are correct when they say that they use pre-screened memory chips to build these modules. Yet, screening DRAMs individually for high-volume products is hardly financially viable (or physically possible), these ICs are not tested manually, but using special testing equipment that can examine memory dies on wafers, chips in groups, and individual ICs.
But for development, debugging, and repairs of memory modules, DRAM chips sometimes have to be picked and replaced manually, which is when companies like Century Micro use special DRAM IC fixtures that look like oversized heat spreaders.
Century Micro is certainly not the only company to use memory module IC test fixtures. Various repair shops use them too, so similar fixtures for standard DDR4 DIMMs are available from retailers like AliExpress for $850 – $880 depending on how many you buy. While for obvious reasons these are used primarily for development, debugging, and repairs, they can also be used to literally hand pick the best DRAM ICs to build memory modules with the highest overclocking potential. Obviously, to do so you need to have access to high-quality DIMM PCBs (or equipment that builds them) as well as a handful of memory chips.
Whether it makes sense to build a custom memory module using hand-picked ICs for overclocking is a different question. Makers of high-performance memory modules are very good in what they do with their testing methodologies that ensure both high data transfer rates and longevity. Furthermore, while high-quality PCBs and pre-screened DRAM ICs used by companies like Corsair or G.Skill are a must, in many cases motherboard design and memory controllers are the real limiters for overclocking, which is why all records are set on high-end motherboards and CPUs cooled down using liquid nitrogen.