Free markets mean any market participant can buy anything. Just like you log on to Robinhood and buy stocks or bonds, one of those users is the Government of Japan.
This is really only slightly simplified as they wouldn't use Robinhood, but they could. Convenience improvements work for all market participants.
Synonyms to save you time understanding this market:
Government bonds = sovereign debt = (when US federal bonds are the context) US treasuries aka US treasury bonds
Fixed income = the market of bonds = credit markets
Junk = not "investment grade" bonds = high yield, but doesn't mean "junk, bad investments"
This is really only slightly simplified as they wouldn't use Robinhood, but they could. Convenience improvements work for all market participants.
Synonyms to save you time understanding this market:
Government bonds = sovereign debt = (when US federal bonds are the context) US treasuries aka US treasury bonds
Fixed income = the market of bonds = credit markets
Junk = not "investment grade" bonds = high yield, but doesn't mean "junk, bad investments"