Initial creation of reserves
The current international monetary system based on pure fiat currency is a post-1970s system. For a long time before that most of the international monetary system was based on the gold standard. With the gold standard, governments issued currency relative to the amount of gold they held or they pegged their currency to another currency that was backed by gold. Hence, the bootstrapping issue you point out wasn't an issue. Note while current central banks often use government securities and bonds as assets to back the issuance of their currency, there is no rule that the asset needs to be a security (it could be gold like before) or that there even needs to be an backing asset at all (at risk of inflation).