Question
How do minnows get in a backyard pond when you don't put them there?

Answer
There are a number of possibilities.

They could come in as eggs or fry attached to or in any plants, water, etc. added from another pond.

They could come in as eggs attached to the bodies of mammals and birds such as the predatory raccoons and herons.

They could come in as adult fish dropped for the mouths or beaks of the same sorts of predators.

A person might have put them in when you weren't looking.

A single shiner of some sort just appeared in my 1800 gallon pond.  From where?  Who knows.

Enjoy your minnows!