Both.
They're basically a layer of "magic rubber sponge" on top of a traditional tire material. That rubber loses the flexibility and plasticity that makes it magic over a few years, and is also easy to wear down in warm weather.
A few thousand miles of warm weather driving will pretty much trash the magic layer, and a few years of time does the same.
They're great, while they work, but just assume a fairly short service life on them. They're still far better than all seasons once worn down, but they don't grip nearly as well as when new.
The studded kind are just hard rubber, open tread patterns, and studs, so they last based on miles driven.