It doesn’t tell me what to do when my iPhone 6 does not detect my iPad 3 . I have them on same wi fi with b/tooth on

You can't do that.

1. Casting is a Google only feature, iPhones do not Cast except in 3rd party Apps that support Casting, like the Youtube app. And will only Cast to Cadt enabled receivers such as TV's, Blu Ray players or a Chrome Cast device.
2. iPhones can Airplay. But you cannot Airplay from one iOS device to another its not a supported function between iOS devices. You can only Airplay from an iOS device to a supported receiver such as an Apple TV box, or an Airplay 2 compatible TV.
3. There is no real point. Anything the iPhone can play, the iPad can play too, so why do you need to Airplay anything to the iPad? Why can't you open it directly on the iPad instead?

What exactly are you trying to accomplish?

How are you attempting to have them detect each other?

Do note that they won't pair or connect over bluetooth directly.

So what exactly are you trying to do?

Hello. This is from Apple’s support pages:

Mirror your iPhone, iPad, or iPod touch

Use Screen Mirroring to see the entire screen of your iOS device on your Apple TV or AirPlay 2-compatible smart TV.

1. Connect your iOS device to the same Wi-Fi network as your Apple TV or AirPlay 2-compatible smart TV.

• • Open Control Center:On iPhone X or later or iPad with iOS 12 or later: Swipe down from the upper-right corner of the screen.
  • On iPhone 8 or earlier or iOS 11 or earlier: Swipe up from the bottom edge of any screen. 

1. Tap  Screen Mirroring.
2. Select your Apple TV or AirPlay 2-compatible smart TV from the list. Need help?
3. If an AirPlay passcode appears on your TV screen, enter the passcode on your iOS device.

Your TV uses your iOS device's screen orientation and aspect ratio. To fill your TV screen with the mirrored iOS device screen, change your TV's aspect ratio or zoom settings.

To stop mirroring your iOS device, open Control Center, tap Screen Mirroring, then tap Stop Mirroring. Or press the Menu button on your Apple TV Remote.

Here is the link to the article: https://support.apple.com/en-us/HT204289#iOS

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Looks as though you can’t mirror from iPhone to iPad.