How do I STOP HomePods from playing sounds from my iPhone?

It started with a simple command: "Siri, play NPR."

Siri, on the HomePods in my living room, did their thing and played NPR using the NPR app on my phone. (Something I don't like.)

A few minutes later, "Siri, stop," and it stopped playing.

A few minutes after that, I played a video on my phone. No sound came from my phone. Turns out, it's playing on my HomePods. The next room over.

I can't figure out how to stop the HomePods from being the default output sound source from my Phone. I'm just trying to listen to a video on a browser, and I can't figure out how to just play on my phone: Can't use the Home app, there's no setting in the Control Center, and I can't find a proper setting in Settings.

How do I STOP the HomePods from playing the sound from my phone by default? The only solution I've had is to reboot my phone.

Thanks in advance.