30 Yr Fixed Mortgage Rates

Is a 30 yr fixed-rate Mortgages charge interest rates based on monthly or annual balance sheet?
Someone trying to tell me my Mortgage interest is payable from the principal balance of the year and not what the balance per month? Is it true?
If you remember when you signed your mortgage papers (or get them out and look at them), there are two interest rates included. One of the rates are the daily / monthly, the second is the annual ….. questions such as these should be made BEFORE papers are signed.