No, they are not security updates, they are full “second number” releases (18.x and 15.x). They fix things, they add features. The detailed list of fixes, as usual, doesn’t exist. The only list available is vulnerabilities, and the only reason it’s available is because vulnerabilities scare users, fear triggers behavior that otherwise would not take place, and the fastest users update, the better business for Apple (supporting older versions cost them money).
They don’t publish the detailed list of real fixes because that’s the trend.