Many Dell systems these days use motherboards from Foxconn, who also supply [different] product to Apple, Cisco, etc. Again, if you want an enthusiast system you won’t want a Dell one, because the CPU, memory, etc. timings are locked at the conservative Intel recommended values. You could cross-flash the board back to the generic Foxconn / whatever model, but why bother when you can get an enthusiast board in the first place and avoid the hassle? Older Dell systems often used Intel boards, and occasionally complete chassis. The XPS D/R/T systems used various Intel SE440BX boards. The PowerEdge 350 was an Intel ISPxxxx server - I forget which exact model.
Regarding memory, the big-name system brands will be using some brand (Hynix, Elpida, etc.) of chips on a module base from that same brand, with the overall part number also carrying that brand. That part number will probably be impossible to obtain elsewhere, unless you’re looking to purchase 10’s of thousands of modules. Any small quantities you find will likely be pulls or spares from Dell or whoever spec’d them. Due to the volatility of memory pricing, memory purchased later (from the same system manufacturer) or supplied as a warranty part, will likely be a different brand.
Module fabricators like Kingston, Smart Modular, etc. build a module base board and install different brands of chips on it, and give it an overall Kingston model number and an internal Kingston part number. Depending on the target for that particular module (specialty OEM, generic spec ValueRAM, etc.) different modules with the same Kingston model number may or may not always have the same brand of chips mounted. Only boards with the same internal Kingston part number will always have the exact same chips on them.
There are benefits and tradeoffs with either approach. During the early years of the PC memory market (up to around the 72-pin DIMM era) there were a lot of fly-by-night outfits that stuck whatever chips they purchased as floor sweepings on a base board (where they probably copied the design from a well-known board), made a half-hearted attempt to test them (if you were lucky) and sold them at inflated prices. This was also the period where those vendors built boards with so-called “logic parity”, because it was cheaper to fake the parity data than to include actual memory chips for parity. These days, the memory market overall has much higher quality. And there is no longer the perception that modules with chips from a RAM manufacturer placed on a base board from an integrator (such as Kingston) are of lower quality - in fact, many people feel that brands like Kingston are superior to modules assembled by a chip manufacturer.
If you purchase a “premium” motherboard from a manufacturer like Supermicro, you’ll find that their “tested memory list” for the board (particularly newer boards) contains mostly (or entirely) chip maker-built modules which aren’t available in the retail channel. So building a system with that motherboard will probably entail using a module that isn’t on their tested list, but which will probably work fine nonetheless.
I’m familar with Dell’s memory because I purchase a fairly large number of systems from them. I have direct experience with Kingston and Smart Modular as I have used both of them to build custom modules for my systems.