Yes, I know there is an actual difference vs. dedicated GPUs with their own VRAM. I say it's marketing because Apple popularized the unified memory term even though, as you said, it existed in iGPUs long before Apple Silicon and was called shared GPU memory.

> I don’t know if expandable memory is possible

It technically is. These new systems (mostly) get their high bandwidth by using more channels (wider bus) of normal RAM modules. A system that has LPCAMM2 sockets should allow using the same LPDDR5X memory but you'd need a socket per two channels. A typical PC only supports two channels so having four (two sockets) would double the bandwidth.