Multi product wafers are a thing, especially on older nodes. It’s accessible to hobbyists
Node size and price / sq mm? The last time I looked at what it would cost to have a partial wafer it was > $25,000 (of which slightly more than half was NRE charges) but would love to find something "hobbyist accessible." !
My understanding is that a chip design for one fab isn’t portable to another.
How does can hobbyist design be just be slapped onto a wafer (with others’ designs) and call it a day?
Indeed, you develop using a fab-specific PDK. But if you share a (Tower Semi, for instance) PDK, your device can have shuttle wafers with multiple designs from different companies. This is done for old nodes and low-volume or RnD parts.