How do I connect my iPhone and MacBook Air using the iPhone mirroring app?

Do your devices meet the System Requirements for this?

iPhone Mirroring: Use your iPhone from your Mac - Apple Support

❝ iPhone Mirroring system requirements

iPhone Mirroring can be used with only one Mac and one iPhone at a time. Your devices must meet these requirements.

• Your computer is a Mac with Apple silicon or Mac with the Apple T2 Security Chip and is using macOS Sequoia 15 or later.
• Your iPhone is using iOS 18 or later.
• Your iPhone and Mac are signed in to the same Apple Account using two-factor authentication.
• Your iPhone and Mac have Bluetooth and Wi-Fi turned on.
• Your iPhone is locked and near your Mac. It can also be charging and using StandBy.
• Your Mac is not sharing its internet connection or using AirPlay or Sidecar. ❞

What are you trying to do?

iPhone mirroring lets you operate many iPhone features from your Mac.

iPhone Mirroring: Use your iPhone from your Mac - Apple Support

Other ways of getting the two to work together include

• Synchronization through iCloud
• Synchronization through shared e-mail accounts
• Other Continuity features (Continuity features and requirements for Apple devices - Apple Support)

I too saw the "Not using Airplay" stipulation and assumed that I wouldn't be able to use the mirroring because I play music from my Mac through my hifi via Airplay. Mirroring my iPhone on the Mac works fine even when I'm using Airplay for music so I assume that the limitation is either only applicable either when you're sending video or when the Mac is an Airplay receiver.