It is - anything short of HBM can be replaced that way.

The issue, of course, is just how much time, effort and skill is required to do it.

It might become a viable commercial process outside third world countries if RAM prices spike even more - or if someone finds a way to teach an off the shelf $1000 robot arm advanced autonomous microsoldering.

Then there's the problem of forcing the device itself to recognize the new RAM. It's a device-specific trick. RAM sticks have a little tag chip that tells a PC "how much RAM this stick has", but RAM chips themselves don't - so the device has to infer its RAM capacity from elsewhere. Like strap resistors on the board. Which you have to find and adjust for the new RAM capacity. That, or fix the RAM values in firmware, which is also device-specific.

If you live in Shenzhen, it's a subway trip away. Good repair services elsewhere can do it too. Either way any warranty you have is probably getting called into question after that, even if that's supposed to be illegal in some places (e.g. the US - they're supposed to prove that your modification/repair is what caused the warranty fault, but in practice…).

Yes, of course. It says - ball grid (array) in the article. Doing it by hand, e.g. with just hot air is proper hard. It's not hard with specialized equipment (that cost way more than the laptop, itself). Sourcing the chips is not easy either.

As for the videos and the Macs, you can see the same people (likely) soldering RAM as well.

On YT there’s dosdude soldering ram on M series SoCs

Good BGA experience and a steady hand seems required

Of course it is possible. Unless you're dealing with manufacturer who actively works against that, like Raspberry Pi.