Radio stations can broadcast multiple feeds now. So most stations have a primary broadcast and sometimes multiple “hd” broadcasts of different formats.

“HD” is a dumb name because it literally means nothing. They named it that when HD tv was a buzz. But it just refers to the ability to broadcast multiple digital signals from basically the same frequency

It’s a newish technology. Most all cars in the last 5 years or so will have HD radio technology. Many since 2012 have had it though