how can I connect my signia hearing aids to my MacBook Air 2019 macOS Monterey 12.6?

MacBook and MacBook Air are macOS while iPhone is iOS. Two different operating systems. There are hearing aid companies the have Support for MFi and standard bluetooth in their hearing aids. You can give your feedback to Apple concerning MacBook and MacBook Air having support for MFi hearing aids on the Apple Feedback link: Product Feedback - Apple

MacBook Airs' do not support MFi hearing aids. MacBook Air will support a standard bluetooth connection. You are going to need a device made by Signia like StreamLine TV or StreamLine Mic. They both will talk to your hearing aids and to your MacBook Air like a bridge between your aids and you MacBook Air. Talk to your audiologist to help you select the correct device.