Federal courts can order someone released from state custody using habeas corpus. If the state court refuses to hold a retrial after a federal court tells them

Federal courts can order someone released from state custody using habeas corpus. If the state court refuses to hold a retrial after a federal court tells them to, the federal court will order the person released. If the state prison/jail refuses to obey a federal court order, in theory the US Marshals turn up and use physical force to make the state officials free the person. Of course, that never happens, because state officials aren’t so dumb as to refuse to obey a federal court order. Also, if they did, they could end up personally facing federal criminal charges.