In stock Android Oreo and below, the volume rockers change ringer volume by default unless audio is currently playing. In order to adjust media volume when media isn't playing, you have to tap the down arrow next to the ringer volume slider that appears at the top of the screen to see the option. Now, Android P has flipped things around, giving media volume the limelight.
On a Pixel phone running the Android P developer preview, when you hit either the volume up or volume down button on the side of your device, you'll see the volume box pop up on the right side of the screen, and "Media" will be the sound that adjusts with the presses. You can tap on the note symbol to mute or unmute. Also, you can tap on the arrows icon to switch your Bluetooth output.
As for your ringtone, there's now the "Calls" toggle under the "Media" slider. Tapping that will rotate it between "Calls," "Vibrate," and "Mute," similar to how tapping the same icon in ringer slider that appeared at the top in older Android versions. Long-pressing on the "Call" toggle will open up the "Sound" menu in Settings so you can adjust the ringer volume, as well as alarm volume and the same old same old.
Depending on how you use Android, this new feature may be a blessing or an annoyance. If you're the former who spends most of their time watching YouTube videos and listening to music, instant access to media volume is much appreciated. If you're the latter, and you get a crazy amount of phone calls, you might rather prefer ringer volume first like it was before, where media volume was only adjusted if you were actually playing media.