The first step is very simple:
https://developer.apple.com/documentation/xcode/creating_a_mac_version_of_your_ipad_app?language=objc

• Found issues:

  • CLLocationManager
    [ ] monitoringDidFailForRegion CLBeaconRegion - no Beacon monitoring

• Solved/Worked Around issues:

  • CLLocationManager
    [x] maccatalyst does not support CLLocationManager startRangingBeaconsInRegion and other Beacon ranging methods. Excluded
    [x] maccatalyst does support new initializer CLBeaconRegion. Updated
  • UIKit
    [x] maccatalyst does not allow to use UIAlertController without actions (and termiation by dismissViewControllerAnimated). Workaround applied
    [x] maccatalyst MKMapView does not allow to zoom out. Added Zoom Controls
    [x] maccatalyst UITableView does not support left swipe actions. Added Edit/Done Button, another work around is to use 2 fingers to swipe left
    [x] maccatalyst does not support UIApplication statusBarStyle. Excluded
  • Miscellaneous
    [x] maccatalyst bundles have a different structure than in iOS. Need to use NSBundle URLForResource
    [x] SocketRocket needs some #ifdef (about to replace SockeRocket by native Websockets)
    [x] error: Today extensions are not available when building for Mac Catalyst. Ignored
    [x] maccatalyst does fail in CLLocationManager startUpdatingLocation - needs Wifi!
    [x] maccatalyst life cycle is different. Need to re-connect after laptop wakes up from sleep
    [x] export Settings/Waypoints does not work --> not yet implemented in Mac Catalyst
    [x] AVCaptureDevice *captureDevice = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo]; returns nil, internal QR code reading removed