How does the Homepod decide where to play music from?

I'm sorry if someone already asked this, but I just bought a new pair of HomePods second-gen today, and I'm totally confused about how they work.

So, right now the HomePod is playing music. However I can't set the volume with my buttons, and my keyboard's clicking sound comes from my phone's speaker. I can control the music from the control center tho. 

What is happening? If I start music with my phone or with my phone's Siri I can control it, but if I ask the HomePod to play it I am only able to control it limited. If the music is streaming on the HomePod, how do I gain control over it on my iPhone?