Are motherboards soldered?

The components are fitted to the top side of the board with their pins protruding through pads on the bottom side of the board. The solder adheres to the board wherever it’s free of solder resist, thereby making mechanical and electrical connections between the component leads and the pads.

Is the GPU soldered to the motherboard?

Especially in non-gaming laptops and even some gaming laptops, the GPU is surface soldered into the motherboard. Not all GPUs are the same size. They are based on different architectures. You need to make sure your slot is compatible with the new mobile GPU you plan to use.

Can you Unsolder a CPU?

No. The motherboard and chipset does not know how to handle a different CPU. Replacing a soldered on CPU cannot be done by hand.

How are motherboards soldered?

They are soldered directly using a fluid paste comprised of a mixture of flux and solder metals. After being put in an oven, the paste hardens and everything is set to place.

Is it possible to replace a CPU without replacing the motherboard?

Yes it is possible, with the right (expensive) tools, but probably not worthwhile. That takes a lot of micro surgery in order to de-solder the old CPU and then solder the new one back on the motherboard.

Is it possible to desolder a CPU at home?

Probably not. It is unlikely you can desolder a 1000 plus pin BGA (ball grid array) part with stuff you have at home. You need a hot air desoldering station with a particular nozzle to fit around the CPU and to not cook everything else nearby.

Is it possible to build an old-style computer like Altair?

Modern CPUs are built on a silicon wafer chip, in an extremely clean environment, using lasers, not soldering. So I guess if you want to build an old-style computer like the Altair or other kit-based computers, sure.