> I guess they're just welding the memory to the CPU chip, but still curious.

Unified memory is more of an architectural and performance characteristic, and does not imply much about the physical layout of the machine. Most unified memory PCs not from Apple don't have the memory on the same package as the SoC. For stuff like AMD Strix Halo and NVIDIA DGX Spark, it's just standard LPDDR packages soldered on the motherboard in the general vicinity of the SoC, and the only difference from mainstream laptops for the past decade+ is that the memory bus is twice as wide.

Yes. The memory is just located very close to the cpu with wires "welded" directly to it. This allows the memory to be run as fast as possible but it's still a RAM component.

The cache parts of memory are on the CPU itself but they are on the order of MB not GB.

They are usually the same family, LPDDR is used for amd and macs, but the fabs are the same as the most expesive HBM memory, if they have a choice they are going to produce the ones that they can sell for more $$.