Your friend states that your mortgage is "an annuity where you are the bank."What does she mean?
When your friend says that your mortgage is "an annuity where you are the bank," she is using an analogy to describe the nature of a mortgage.
An annuity is essentially a series of periodic payments made over a specific period of time. In the context of a mortgage, it refers to the monthly payments you make to the bank to repay the loan you borrowed to purchase your home.
Comparing it to being the bank means that you, as the homeowner, are acting like a bank in the sense that you are lending money to yourself by taking out a mortgage. The bank provides you with a lump sum amount upfront to purchase the house, and in return, you have to pay them back with interest over a specific time period.
Just like a bank earns interest on the money it lends, you, as the homeowner, pay interest to the bank for borrowing their money to buy the house. So, in a way, you are assuming the role of the bank by making regular payments, and the bank earns interest on the loan.