How do you disconnect iPhone from apple carplay ?

Unplug it...? Not sure what you mean by "disconnect".

If you no longer want it to be allowed to connect to a specific vehicle, go to Settings>General>CarPlay. Select the appropriate vehicle, and tap "Forget This Car".

1. Before you get in the car, open the Settings app.
2. Tap General > Restrictions.
3. Select Enable Restrictions.
4. Enter a passcode.
5. Repeat that passcode.
6. Toggle the CarPlay switch to Off.

(if you don't want it to "autoplay" anymore when you start your vehicle up)

In your car you can also click on "Forget this device"

In my car I just select Radio or Media (SD card option) to switch from CarPlay. That will keep my phone charging but not necessarily be using the CarPlay features. Yes, it does automatically connect to CarPlay but with the touch of a button I'm no longer connected until I select it again.

Restrictions has been moved and is now part of Screen Time.