Queen Elizabeth II is the queen of 54 Commonwealth countries. 16 of these countries are called Commonwealth Realm countries and Canada is one of them.
When Elizabeth steps down or dies, her son, Prince Charles will become King. If he steps down or dies, his first-born son, Prince William will become King–even though Princess Anne is the Queen’s only daughter and is next in age to Prince Charles.
In other words, because she is female, the crown wouldn’t go to Princess Anne.
Since the beginning of the British monarchy, men have been chosen first to become the next ruler.
A woman can only be chosen when there are no men in the monarch’s direct line, (like a brother or a brother’s sons).
That is how Elizabeth became queen. Her father, King George VI, had two daughters and no sons.