Meanwhile it's just everyday business in China. People send their laptops or graphic cards with soldered RAM and get the device back with double RAM/vRAM. "Brother Zhang" is one of those who operate such a shop: https://youtu.be/TcRGBeOENLg
Had such services been more available in the US, I would not have needed to sell a laptop that had 8GM RAM and could no longer handle my workflows but was perfect otherwise.
The only thing approaching "secret sauce" is the BIOS modding, which varies in difficulty depending on the device. For many devices it's just a case of reconfiguring strap resistors.
This can be a result of unexpected and costly if not worse, surprises.
Racking in money printed out of thin air by banks using a sleight of hand then forcing people to give us things in exchange for said printed money under threat of being bombed, invaded, and sanctioned
Unless you can put money on your words and provide some sort of guarantee, nobody is going to take that.
In practice, it's more like a 75-90% failure rate if you're unfamiliar with ball-soldering techniques, which most are since it's been considered impractical to do soldering in the US since the through-hole days, and so there's not a professional tradesman class to teach kids. What does exist of soldering is stuck in the far past from an electronics manufacturing perspective.
Tl;dr: https://xkcd.com/2501/
Hot air station, stencil, stencil frame. preheater, etc. Even board view sometime need to be purchased.
I know that people like to argue "but I can't upgrade RAM". Yes. But as we see here not impossible. Your car also needs some effort to change some parts and perhaps even more effort compared to resoldering some chips at a shop that knows how-to.
How is this relevant when the same point applies to system RAM?
512 conductors that have to be exactly the same length. Make them longer you'll say- now you just lost your high memory clocks to propigation delay and parasitics.
Seeing the problem?
Not to mention that the bar of acceptibility is moving. We had RAM in slots in laptops for a long while, but it's (not so) slowly going away. You can still find laptops with SO-DIMMs, but it's not a given anymore.
Go back far enough (486) and cache was off-chip on simms too!
They usally came in 8 to 16 discrete chips with pins on both of their sides, to be soldered on board, or plugged into sockets.
Plus one or two 'tag-rams' as chips of the same size.
All depending on the chipset, its cache-controller, and which capabilities the board-maker decided to use/expose/support. Which sometimes the BIOS would get wrong, still :-)
Good luck finding a motherboard which will run at those speeds reliably. Sure, you can find a board which says it will do it, and it will sometimes manage those speeds, if you also have a CPU with a memory controller that can handle all the interference. When you solder the chips and thus make the traces much shorter and less susceptible to interference, that problem largely goes away.