Using IPhone with HomePod with a Music app

I teach music lessons. I regularly use an app that slows down songs for my students. How can I use that app to play music so that it comes through my HomePod?

in fact, how can I control the song being played - or my audiobooks - using my iPhone XR rather than commands through the HomePod?

I am not the most tech literate person so please keep it simple. 😉😉

thank you!