Afaik they use the same type of memory as phones do, variants of LPDDRx. “Unified” does not denote a different type, it means that both the GPU and the CPU can use the same memory (similarly to integrated GPUs using the main system ram).

Nah, it’s some kind of a mobile GPU.

How is that card even used if it doesn’t have a number? Can you use it only physically with payment terminals? I have a debit card and it can be used just like credit card but without the loans part obviously.

Afaik never (or at least not with the current versions) because HBM has to be directly connected to the GPU or CPU die via an interposer, it’s not something you can slot into a mainboard.