Money is a man-made construct, invented as a substitute for and a representative of time and work. There is as much money as whoever issues it decides, but the value of it, and how much work and time it represents, depends on the relationship between how much there is and how much time and work are represented.
I am sure someone else can answer the more modern side of this question better than me, but the historic side of "Where does money come from" goes something like this.
Once upon a time, many centuries ago, no-one had any money. Everyone lived in families or small groups, hunted or foraged for food, made a shelter from whatever they could, and made warm clothes from the skins of the animals they had eaten. The children and the old and sick received food from those who could hunt or forage, but as soon as they were able, children joined in first the foraging, then the hunt. When food was plentiful, everyone ate well, when it was scarce, they didn't and the very young and the old and sick died. If it was possible, surplus food was stored for the famine months. In a small community, everyone knew everyone else and was aware that if they shared their food when they had it, others would share what they had too, and they would all benefit.
Later, some people specialised in one particular skill - making arrow-heads for hunting, weaving cloth. While they were doing these things, they were less able to hunt for their pown food. Those who needed arrow-heads or cloth shared their food with the artisan. Travelling tool-makers exchanged their goods for board and lodging. Certain individuals were very good at singing and telling stories. They travelled from one place to another, and were welcomed for their tales and music, and as visitors were fed well in return, before they moved on.
As communities became larger, specialisation became more common, until there were people who never hunted or foraged at all, but whose own skills supported them when exchanged for food or the products of the skills of others. The whole trade system grew more and more intricate, with an "exchange rate" emerging for the equivalent value of, say, a cloak against a stone axe against free food for the winter. With the arrival of the even more specialised skills of metal-working (which was usually treated as a secret magic, and the workers as wonder-workers) the different values of different products became even more pronounced.
In several places, a system of symbols was invented. Something rare and hard to find came to represent a certain value of barter. Cowrie shells, for instance, were not very common, were small, easily recognised, impossible to forge or adulterate, and could be stored away when not needed to pay for something, unlike a spare joint of venison. Gold and silver ornamental jewellery could be broken up and exchanged for goods or services.
The usefulness of these symbols was their rarity, and their significance was that they were indeed symbolic.
If there were only two hundred cowrie shells in existence in the area, then the value in shells there of any artifact or service could only be the proportion of that two hundred that the atrifact or service represented in relation to everything else that could be bought there. Someone finding a hundred more cowrie shells on the beach might feel briefly as rich as Croesus, but he couldn't buy any more with it than the area produced - and he would find that if there was no more of it there to buy the price of everything went up and the value of each shell went down. He could give some of it to the poor and disadvantaged, but the prices and values would have changed for them, too, so they could end up even worse off than before his generosity.