What is the mechanism by which being the reserve currency sprays a firehose of money into the US? Keep in mind that being the major economy also carries costs

What is the mechanism by which being the reserve currency sprays a firehose of money into the US? Keep in mind that being the major economy also carries costs - the US spends many times as much as other countries on its military and has no realistic path towards spending less. And either way, their economy has been wildly succesful since the 1800’s.