They're not. TB cables are always active, AFAIK. USB-C is a port/connector design, and Thunderbolt 3 just so happens to use a USB-C port and connector. Previous generations used mini DisplayPort.
That is correct.
Putting it simple, USB-C is just a connector design (like the USB "A" and USB "B") that allows both Data and Power to be transmitted, while allowing the manufacturer dynamic data/power ratio (a device can transmit 0.5A@5V or 5A@20V) and the round shape and mirrored/inverted pins also allowing the connector to fit and work properly in both orientations (a godsend, if you ask me).
Now, as far as I know, Thunderbolt is a physical layer standard developed by Intel, and can support many different protocols and uses (Audio/Video, Data, Power). You can pass DisplayPort, HDMI, USB and other protocols through this standard.
Both TB 1.0 and TB 2.0 uses the DisplayPort Mini (which is open, unlike HDMI) as standard connector. TB 3.0 uses USB-C as standard connector.
Intel gave the Thunderbolt license and rights "for free" to the USB Forum, and the new USB 4 protocol will use a implementation of Thunderbolt (which is being called TB 4.0, but have the same specs as TB 3.0) as the physical layer for transportation.
As of now, USB 4 and Thunderbolt 4 are the same thing, and they only gonna use the USB-C connector. The USB "A" (small rectangle) and USB "B" (big square) will not be used in USB4/TB4.