Thunderbolt refers to socket or plug, or both?
Solution 1:
Actually it's neither.
The plug is a USB-C connector.
The socket is a USB-C socket.
Thunderbolt 3 is the specification that details the pin-layout, signals, cable requirements, power and protocols that, amongst other things, makes it possible for those connectors, plugs and cables in the picture to enable the functionality mentioned.
You can read a more in-depth description in this technology brief:
https://thunderbolttechnology.net/sites/default/files/HBD16235_Thunderbolt_TB_r05.pdf
The brief is a bit more technical than just the illustration, but still at a level that can be understood without being a computer engineer.