Which Bootstrap property is applied when the button is clicked?

Question

HTML

Good day.
The essence of the problem is as follows. If we apply the btn property to a button when layout on Bootstrap, then when we click on the button, it gets a certain background color even when we move the mouse away from the element. This property is removed only when another button is clicked. Please tell me how it is implemented and how to give your own properties for such an event / behavior. I am a beginner, I am at a loss in terminology.
Thanks in advance to all who respond.

Answer 1

button:focus {
    //Bla-bla-bla
}