Who pays the network fee?

So, I'm curious about this concept of network fees in the world of cryptocurrency. Could you explain, who exactly is responsible for paying these network fees? Is it the sender, the receiver, or perhaps a combination of both? And how does this process work exactly? Are these fees determined by the cryptocurrency itself, or are they subject to the whims of the market? I'm really interested in understanding the ins and outs of this aspect of cryptocurrency transactions.