>>519578775 (OP)
A 'default' in this case means the US simply wouldn't make bond payments, which would crash the value of those bonds, meaning that the market for US treasuries would tank, along with all the associated funds and stocks which hold those treasuries.
One practical effect would be a substantial contraction of the US federal budget, which relies on deficit spending. This would no longer be possible, since no one would buy US debt securities except for the US itself, which means lots of emergency money-printing.
The federal government would have to triage to pay for things it actually considers important, while probably shedding most entitlement payments (pensions; social security) as well as social welfare systems such as Medicaid.