What, in the most literal sense, are eurodollars? There are readily available &quot;definitions&quot; including on this site, but I want to know what they are in a concrete sense.  Because the definitions make them basically sound like just electronic bank deposits in dollars at a foreign commercial bank, but in a literal sense this seems nonsensical unless those foreign banks (or the central bank in their country) have accounts at the US Fed. After all, to a commercial bank, electronic deposits are just a local accounting fiction, the movement of which gets accounted for at the end of the day by a net transfer of central bank reserves. So are they US treasury securities? or physical cash? or do foreign banks indeed have accounts at the fed?
They actually literary are a deposit accounts of US dollars outside US. Also no those foreign banks will not generally have accounts at Fed. In fact one of the reasons why eurodollar deposits pay higher interest than a dollar deposit in US as they are more risky since they are neither subject to FDIC insurance and those banks will not get liquidity from Fed if there is run on them.

After all, to a commercial bank, electronic deposits are just a local accounting fiction, the movement of which gets accounted for at the end of the day by a net transfer of central bank reserves.

But there is no 1:1 correspondence between amount of dollars in circulation and Fed reserves. You can have a look at Fred data for volume of reserves and money stock $M2$ and you can see that at any point in time money stock is several orders of magnitude larger than volume of reserves.
If you want to know how eurodollars work on conceptual level then have a look at Niehans, &amp; Hewson (1976).
Actually a good primer was also provided by Milton Friedman (1971). Friedman, also includes concrete example.