When you pay taxes, you pay them to the Canadian State. The Monarch is the head of the state. So you are paying taxes to the Monarch.
The Monarch is also the Commander-in-Chief of the military. When you join the military you must swear allegiance to the Monarch of Canada.
"I, [name], do swear (or solemnly affirm) that I will well and truly serve Her Majesty, Queen Elizabeth the Second, Queen of Canada, Her heirs and successors according to law, in the Canadian Forces until lawfully released, that I will resist Her Majesty's enemies and cause Her Majesty's peace to be kept and maintained and that I will, in all matters pertaining to my service, faithfully discharge my duty."