In regard to the purchase of bonds by the central bank I wrote the following paper which is called Financial Instrument Generation in the U.S. Financial System.
Based on the U.S. flow of funds I divide the balance sheet positions and transaction flows into four sectors: central bank (Fed or RBA), Treasury department of the federal government, Banks, and Nonbanks. In terms of double-entry accounting customs every sector has a balance sheet based on a Chart of Accounts. The flow of funds statistics use these principles but did not incorporate the four sector model exactly in their structure.
When a central bank (CB) debits an asset account for an increase it also credits a liability account for an increase which expands the CB balance sheet. When the federal government runs a deficit, is issues net new Treasury securities in the float of federal "debt" (the secure savings of other units) and when it runs a surplus it retires securities from the float by letting them mature without replacement to dispose of the surplus.
Bill Mitchell, an MMT proponent, describes and illustrates currency, reserves, and Treasury securities as financial assets stored in the Non-Government tin shed accumulating at the rate of Government deficits over the history of the nation:
Stephanie Bell Kelton, an MMT proponent, in essence argues that the sale of Treasury securities is simply the payment of interest by the Fiscal agent to help the Monetary authority maintain control over the policy rate of interest.
The MMT position is to consolidate the CB balance sheet with the Treasury balance sheet kept as the fiscal branch of federal government. This cancels the mutual assets and liabilities on the consolidated balance sheet to form a three sector economy with the Sovereign government, Banks, and Nonbanks.
But then inside the Sovereign government there are two cooperative yet distinct operations of the Fiscal agent (Treasury function) and Monetary authority (central bank).
So in terms of your question there are the two operations called fiscal and monetary policy and there are the two kinds of liabilities which pay zero interest (high powered money in the form of national currency and central bank liabilities) or which pay interest (interest on bank reserves and Treasury securities). In general the Bank and Nonbank sectors are given the option to hold the zero interest liabilities or to earn some interest so that the Monetary authority retains control over some policy rate in the money markets. Also if the fiscal authority does not authorize deficit spending then the monetary authority can independently purchase less liquid financial assets from Nonbanks to provide more liquid financial assets to Banks and Nonbanks via so-called Quantitative Easing. The interest paid by the government is just a tool of Monetary policy and is of no great consequence compared to other concerns for managing the affairs of the nation.