How to communicate faster than the system clock
I guess the confusion is you assume you can only send one bit per clock cycle. There are lots of ways a communication scheme can essentially encode more than one bit per symbol. A symbol is abstract idea as the atom of transfer in a communication system. 

It's really too big of a topic to cover in an answer to this question in any depth, but imagine you weren't constrained to binary values and instead could send one of 1024 voltages as a symbol. In effect that would be 10 bits of information per symbol, and you would get 10x the "clock speed" in bandwidth. That's how the old NTSC video encodes data for example. 

Another way of getting that kind of bandwidth is using buffering and then delegating the transport to specialized transmitters and receivers, sometimes called SERDES (for serializer / deserializer) blocks. You can't sustain the throughput that these are capable of without a source or sink capable of keeping up, but you can reduce the latency of the transfer of blocks of information between computing nodes using something like that. Look up Phase-Locked Loops (aka PLLs), as FPGAs and ASICs can use these to derive faster clocks from a basis clock to do this sort of thing.

Another way is simply having lots of parallel channels to transmit the data on. Think of the old parallel ports on PCs, in a single clock you transfer a whole bunch of bits, each bit on its own dedicated wire. USB-C and its kin have a lot more data pins than one RX and one TX for example. 

Bandwidth is an aggregate property involving the net effect of all kinds of techniques like these, because many can be used together even.