Their open market operations affect short term bond yields which in turn affect long term bond yields. They clearly intervene and it's hard to imagine interest

Their open market operations affect short term bond yields which in turn affect long term bond yields. They clearly intervene and it's hard to imagine interest rates being as low as they have been without the intervention. When they buy bonds, it pushes the yields lower, if things were where they were naturally the fed/ecb etc wouldn't have the sheer number of bonds they do on their balance sheet.