Most currencies in the world today are issued by governments. Does this mean
Not at all, things have value not because governments or individuals assign them value but because a significant group of people do. Whatever we are prepared to accept as valuable makes something valuable. Governments certainly cannot do it as we can see at times of hyperinflation when governments say they support the currency but people in general do not so it becomes largely worthless. Even the value of commodities varies from time to time and place to place. It is not directly because of anything officials or individuals do, it is a matter of supply and demand, and fashion.
Moses Maimonides pointed out that what we need to live is bread and water, and these are very cheap, whereas some useless thing like a jewel is often very highly valued. He is right and what makes it valued is nothing objective about it, just how people whose opinion counts feel about it.