Everyone. National debt is basically from issuing Treasury bonds. You buy one with a fixed term, and when that time comes, the government pays you back that amount with interest.
For reliable countries like the US, this is great. Historically, the US has been very good at paying its debts, so buying T-bills is a reliable investment.
Who are they in debt to and who are they paying interest to?
Everyone. National debt is basically from issuing Treasury bonds. You buy one with a fixed term, and when that time comes, the government pays you back that amount with interest.
For reliable countries like the US, this is great. Historically, the US has been very good at paying its debts, so buying T-bills is a reliable investment.