How is bandwidth and frequency exactly related?

Hello,

I don't know if this belongs here but here goes.

I don't see how bandwidth and frequency are exactly related. I'd need a very simple explanation of bandwidth to maybe keep the two apart.

Now in physical terms, free space is used as a transport medium where electromagnetic waves get transmitted. Using frequencies we can establish different communication channels, to be short (a more elaborate answer would go into the different types of multiplexing for this medium)

Bandwidth in short is the threshold for frequencies and the bigger the bandwidth the more data can be transferred or so I understand. One interesting and very simple explanation I heard is in wave length multiplexing the higher the bandwidth the more colors (=frequencies in wave length multiplexing?) are used and therefore the data can be sent across more channels/colors. However if you look at my understanding of frequency multiplexing different frequencies are used for different channels of communication.

Now what I struggle with is this: I understand that different channels can use different frequencies but can different frequencies also be used by one channel to increase data rate? If yes, what context is that used in? Users in a mobile network cell wouldn't start occupying different frequencies to increase data rate, right?