Foreigners sell you things in exchange for your currency, because they want to hold the currency. Once a currency is usable for payment in taxes, it can be analysed like any other commodity.
There are no 'reserve currencies'. Anybody holding a financial asset in a denomination outside its home physical zone area is a 'reserve'. They are operationally no different from hoarding diamonds or gold bars.
The US can effectively tax the world as long as they use their military power to ensure oil producing countries denominate their oil exports in US dollars.
The reserve currency system is a stealth tribute system.