I assumed them. I decided that I wanted arms. I thought a bit about what I wanted, and decided on this. That makes them my arms. That is the way it works in most of the world.
Some places have government offices that register arms.
In a few it is illegal to assume arms on your own authority. In Scotland this law is enforced. But in most of the world, you are on your own. Take whatever arms you like. Manners dictate that you not choose arms already in use in your country.