On some systems ... you use Siri by pressing and holding the voice command button on your steering wheel.
Seems we can use Voice Control commands to interact with CarPlay
Use Voice Control commands to interact with CarPlay - Apple Support
Eg: says:
Turn Voice Control on or off
- In CarPlay, go to Settings
> Accessibility. - Turn Voice Control on or off.
- When you turn on Voice Control,
appears in the sidebar to indicate that Voice Control is listening for commands.
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Start using commands
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To use Voice Control in CarPlay, just say a command.
Voice Control: Say, for example, “Open Maps,” “Pan left,” or “Go back.”
When you say multiple commands in quick succession, pause about half a second between commands.
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Pause or resume Voice Control
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Pause Voice Control when you don’t want words you say to be interpreted as commands. Resume Voice Control when you’re ready for it to listen for commands again.
Voice Control: Say “Stop listening” or “Start listening.”
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Use Voice Control in Music
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If you use Voice Control in the Music app in CarPlay and don’t know what to call a button, you can use the button names in the table below.
If a button has a text label, use the text shown as the name of the button. For example, to tap a button labeled Cancel, just say “Tap Cancel.”
Button
Name

Play

Pause

Fast Forward

Rewind

Shuffle

Repeat

Autoplay

Favorite

More

Go back

List
NB: There's a great deal more info. in the linked article at the top.