Your later questions answer your first question. Fiat currency fails and loses purchasing power because of inflation that is continuously made by governments and central banks.
OK but why is the printing of new money causing a loss of value in a fiat currency and who determines and decides that the currency is now less valuable and that prices will go up? Fiat isnt a free market adopted by the people. What is in the background that determines how much one currency will be worth against a different currency, gold, or bitcoin?
For example, fiat is not doomed to fail, it's been operating quite successfully globally.
I think the word successfully that you used is very subjective. I said doomed to fail because the money gets devalued. Imagine you save for several years to buy an apartment. After a long time, you have €300.000. Happy that you have finally made it, you find out that those €300.000 will no longer be enough as they were several years ago. Now you need €500.000.
The more paper money in circulation, the more it adds cost to the inflation.
Why and how?