In a way, USB Type-C and USB 3.1 are different. Although USB Type-C is a standard part of the USB 3.1, the norm of USB Type-C is used for defining the connector interface,
while USB 3.1 is a transmission standard. As far as consumers are concerned, USB Type-C is the “shape” of the connector, which allows for different transmission standards, like USB 3.1 and Thunderbolt3, to be embedded in it.
When a USB Type-C connector that uses a Thunderbolt 3 standard is connected to an external disk that supports Thunderbolt 3, the transmission speed can reach up to 40GB/s.
Thunderbolt 3 is backward-compatible to USB 3.1 so that a USB 3.1 external disk can connect to a USB Type-C connector interface. But the speed will decrease to 10GB/s (the theoretical speed of USB 3.1).
On the contrary, an external disk that uses Thunderbolt 3 cannot work when connected to a USB Type-C connector with a USB 3.1 transmission standard, because the specification is not supported.