Usually aluminium or glass. There’s a metallic coating applied to the outside surfaces that stores the data. That layer is very thin though, so most of the material is the substrate.

Oh right, that makes sense. I was only thinking of Matter as serving low bandwidth devices but it also runs over WiFi and ethernet so I guess it can do video for security cameras etc. and evidently Casting audio and video also.

Also Matter is the smart home interop standard. Seems close enough for some confusion in what Matter compatible means on a device.