Foreign countries buy USD through trade. Japan needs dollars for international trade so the US Treasury prints dollars and gives them to Japan and in return Japan gives the US Nintendo Switches or whatever. Those dollars go into the international trade system and some of those dollars will just circulate internationally indefinitely and won't ever make their way back to the US, hence the perpetual trade deficit. This is a great deal for the US because at the national level they effectively got those Nintendo Switches for free.