"x=x for all x" - this is true if you mean to say everything on the Fed's balance sheet is 'Debt' ... but that's wrong - it's referred to as 'Assets'. The currency itself is the 'Liability'.
A Central Bank issues currencies for assets, in which case the 'currency' is 'backed by' those assets. Ostensibly, one could trade the currency back for those assets.
That's not generally what we think of as 'debt'.
Now, much of the 'Assets' are actually US TBills, which is 'debt' - but - not all of the Fed's Balance sheet is TBills.
So 'x' does not equal 'x'.
The US Fed Balance sheet [1]
ECB Balance Sheet [2]
Historically the asset would be gold - and that still makes up part of the Fed's balance sheet. Since 2008, the US Fed has issued currency based on mortgage securities, essentially, it owns homes.
Otherwise, the Fed owns TBills, i.e. US Government debt, as assets, which is part of the basis of this article: The US Fed is one of the 'major owners' of TBills.
Yes, it's legit to point out that 'Internal Operations' are relevant and partly why Japan is not having major inflation, but your denotation of 'Internal' as being 'Government Agencies' is not quite right: by 'Internal' what we really would mean is 'internal to the Japanese economy'.
So Japanese debt is held by Japanese individuals and institutions of all kinds, public, private etc.. So 'Internal' means anything in their entire economy.
For a breakdown of a Central Banks Balance Sheet have a look here [3] which breaks down ECB balance sheet growth over the last while.
[1] https://www.investopedia.com/articles/economics/10/understan...
[2] https://www.ecb.europa.eu/pub/pdf/other/ecb.eurosystembalanc...