The extender flex just incorporates a mains socket so that you don't lose a mains socket by plugging the device in. The extender doesn't have the mains socket.
Whichever you choose comes as a pair. the extender which connects to the hub and a slave which you connect your devices to. If you want wifi as well as Ethernet at the slave, you need a 'Hotspot' kit rather than just an extender kit.