Why is the structure of the national debt more important than its volume

For the state, it is not the volume of its public debt in absolute terms that is terrible when it approaches the level of GDP, but the threat of its debt when the volume of external borrowing tends to 100% relative to domestic debts (printed money in its national currency).

Excess cash in your economy with an increase in inflation can then simply be withdrawn from circulation by raising interest rates (everyone will strive to keep money, not spend it) and paying off government securities to the central bank (reducing the volume of lending and financing budget expenditures).

But you can't do that with external debt anymore. If it is impossible to repay your obligations, you will have to pay with national property - property and land, which in fact is default.

Therefore, for example, for the USA, exceeding the ceiling of its of the national debt poses no threat, unlike some other states that are almost 100% dependent on external creditors.