How the Federal Reserve Manages Money Supply I'm trying to understand how the Federal Reserve manages the money supply via open market operations. According to this post, when Fed buys securities, that increases bank reserves, which allows banks to lend more money, which increases the money supply. That makes sense. But when the issuer pays off the bond, wouldn't that reduce bank reserves, thus reducing lending, thus reducing the money supply? It seems like the overall effect of bond buying would be to increase the money supply in the short term but in the long term there may be a slight reduction in the money supply if the bond is paid back with interest.
Please let me know what I'm misunderstanding here. Thanks!
How the Federal Reserve Manages Money Supply?

Federal reserve manages money supply in various different ways (see Blanchard et al Macroeconomics ch4). The main ones are:

By managing interest rates on a reserves: lower interest rate reserves allow banks to lend more and expand money supply and vice versa.
By open market operations: where purchase Fed purchases debt in exchange for new money (which eventually increases reserves as that money finds it way to banks). Issuing new debt increases money supply and repayment of the debt decreases it.
By changing reserve requirement. Currently they are abolished by Fed, but nothing prevents Fed from bringing them back if they desire so. Here higher reserve requirement contracts money supply and lower one expands it.
Other: Fed can also affect money supply through other means such as banking regulation, although that is too broad of a topic to discuss in detail in one SE post. You can find some explanation of that in McLeay et al (2014) and sources cited therein.



But when the issuer pays off the bond, wouldn't that reduce bank reserves, thus reducing lending, thus reducing the money supply?


When the bond is payed off it does lead to reduction in money supply unless the money are lent again. However, money supply is reduced just to the original point. If Fed injected 100 USD to the economy through OMOs and if through fractional reserve banking system that injection multiplied up to 1000 USD then repaying that debt only destroys the 1000 USD not any additional money (e.g. if M before injection was 1 billion USD after repayment it will again be just 1 billion USD).

It does not reduce money supply further than it was before it was expanded. The interest rate is paid to the bank. In case of commercial banks they keep the money as a profit (after paying for all costs of course) so it will still be circulating in the economy. In case of US government paying interest to Fed, again money supply is not additionally destroyed because Fed is government institution and it sends all profits from interest back to the treasury (e.g. see WSJ article about profits they send back in 2020) so the money used to pay interest stays circulating in the economy.