SSD controller is on the same die, but the actual SSDs in a Mac Studio are slotted and technically upgradeable. Storage bandwidth generally matches NVMe drives of the same generation, as Apple is ultimately using the same NAND modules over PCIe.

My ideal Mac Studio would have an NVMe slot or two for additional internal storage, and those slots could be accessed without opening the case. But Apple won't do that, as convenient internal storage upgrades would likely lower their profits.