Are the pistachios already opened when they're not roasted or are they closed and they open with heat just like mussels?

Unroasted pistachiOS are typically still closed and require heat to open, similar to mussels. During roasting, the heat causes the shells to expand and crack open, making them easier to access. In contrast, if you find pistachiOS that are already opened, they are usually roasted and salted, and the opening is a result of the roasting process.