The files you will need to follow along the creation of the "Chooser" example app from my MacSysAdmin Online 2021 presentation.

ChooserUI.mobileconfig

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>PayloadContent</key>
	<array>
		<dict>
			<key>PayloadContent</key>
			<dict>
				<key>com.scriptingosx.ChooserUI</key>
				<dict>
					<key>Forced</key>
					<array>
						<dict>
							<key>mcx_preference_settings</key>
							<dict>
								<key>urlScheme</key>
								<string>mailto</string>
							</dict>
						</dict>
					</array>
				</dict>
			</dict>
			<key>PayloadEnabled</key>
			<true/>
			<key>PayloadIdentifier</key>
			<string>MCXToProfile.1c6b1ed7-d170-4c11-9950-2da5d7cca247.alacarte.customsettings.8df89e15-83ba-4b94-bc91-c9422f41439c</string>
			<key>PayloadType</key>
			<string>com.apple.ManagedClient.preferences</string>
			<key>PayloadUUID</key>
			<string>8df89e15-83ba-4b94-bc91-c9422f41439c</string>
			<key>PayloadVersion</key>
			<integer>1</integer>
		</dict>
	</array>
	<key>PayloadDescription</key>
	<string>Included custom settings:
ChooserUI

Git revision: eacd7a583e</string>
	<key>PayloadDisplayName</key>
	<string>MCXToProfile: ChooserUI</string>
	<key>PayloadIdentifier</key>
	<string>com.scriptingosx.ChooserUI</string>
	<key>PayloadOrganization</key>
	<string></string>
	<key>PayloadRemovalDisallowed</key>
	<true/>
	<key>PayloadScope</key>
	<string>System</string>
	<key>PayloadType</key>
	<string>Configuration</string>
	<key>PayloadUUID</key>
	<string>1c6b1ed7-d170-4c11-9950-2da5d7cca247</string>
	<key>PayloadVersion</key>
	<integer>1</integer>
</dict>
</plist>

LSApp.swift

//
//  LSApp.swift
//  LSApplication
//
//  Created by Armin Briegel on 2021-08-30.
//

import AppKit


/**
 An LSApp object represents an application on disk for use this with the LSKit functions.
 */
struct LSApp : Identifiable, Equatable {
    /** local file URL of the app*/
    let url: URL
    
    /** the app's bundle object*/
    let bundle: Bundle
    
    /** app bundle identifier */
    let identifier: String
    
    /** app name (from bundle)*/
    let name: String
    
    /** path to the app */
    var path: String {
        return url.path
    }
    
    /** returns the identifier */
    var id: String {
        return identifier
    }
    
    /** app icon (256x256) */
    var icon: NSImage {
        let icon = NSWorkspace.shared.icon(forFile: url.path)
        icon.size = NSSize(width: 256.0, height: 256.0)
        return icon
    }
    
    /** returns whether the app is the default handler for the url scheme */
    func isDefault(for scheme: String) -> Bool {
        return LSKit.defaultURL(for: scheme) == self.url
    }
    
    /**
     default initializer
     - Parameter url: local file URL to the application
     */
    init?(url: URL) {
        self.url = url
        guard let newBundle = Bundle(url: url) else { return nil }
        self.bundle = newBundle
        self.identifier = self.bundle.bundleIdentifier ?? ""
        self.name = self.bundle.infoDictionary?["CFBundleName"] as? String ?? ""
    }
      
  /** LSApps are equal, when the identifiers are equal */
  static func == (lhs: LSApp, rhs: LSApp) -> Bool {
    return lhs.id == rhs.id
  }
}

/**
 adds functions to LSKit that return LSApp objects instead of URLs
 */
extension LSKit {
    /**
     gets a list of applications for the given url Scheme
     - Parameter for: the url scheme (e.g. "http")
     - Returns: Array of LSApp objects
     */
    static func applications(for scheme: String) -> [LSApp] {
        var applicationList = [LSApp]()
        let urlList = urls(for: scheme)
        for appURL in urlList {
            if let newApp = LSApp(url: appURL) {
                applicationList.append(newApp)
            }
        }
        return applicationList
    }
    
    /**
     gets the default application for the give URL scheme
     - Parameter for: the url scheme (e.g. "http")
     - Returns: the default application as an LSApp
     */
    static func defaultApplication(for scheme: String) -> LSApp? {
        if let defaultAppURL = defaultURL(for: scheme) {
            return LSApp(url: defaultAppURL)
        } else {
            return nil
        }
    }
}

LSKit

//
//  LSKit.swift
//  LSKit
//
//  Created by Armin Briegel on 2021-08-30.
//

import Foundation
import AppKit

class LSKit {
  
  /**
   returns a list of URLs to applications that can open URLs starting with the scheme
   - Parameter scheme: url scheme (excluding the `:` or `/`, e.g. `http`)
   */
  static func urls(for scheme: String) -> [URL] {
    guard let url = URL(string: "\(scheme):") else { return [URL]() }
    
    if #available(macOS 12, *) {
      //print("running on macOS 12, using NSWorkspace")
      let ws = NSWorkspace.shared
      return ws.urlsForApplications(toOpen: url)
    } else {
      var urlList = [URL]()
      if let result = LSCopyApplicationURLsForURL(url as CFURL, .all) {
        let cfURLList = result.takeRetainedValue() as Array
        for item in cfURLList {
          if let appURL = item as? URL {
            urlList.append(appURL)
          }
        }
      }
      return urlList
    }
  }
  
  /**
   returns URL to the default application for URLs starting with scheme
   - Parameter scheme: url scheme (excluding the `:` or `/`, e.g. `http`)
   - Returns: urls to default application
   */
  static func defaultURL(for scheme: String) -> URL? {
    guard let url = URL(string: "\(scheme):") else { return nil }
    
    if #available(macOS 12, *) {
      //print("running on macOS 12, using NSWorkspace")
      let ws = NSWorkspace.shared
      return ws.urlForApplication(toOpen: url)
    } else {
      if let result = LSCopyDefaultApplicationURLForURL(url as CFURL, .all, nil) {
        let appURL = result.takeRetainedValue() as URL
        return appURL
      }
      return nil
    }
  }
  
  /**
   set the default app for scheme to the app with the identifier
   - Parameters:
    - identifier: bundle id of the new default application
    - scheme: url scheme (excluding the `:` or `/`, e.g. `http`)
   - Returns: OSStatus (discardable)
   */
  @discardableResult static func setDefault(identifier: String, for scheme: String) -> OSStatus {
    if #available(macOS 12, *) {
      // print("running on macOS 12, using NSWorkspace")
      let ws = NSWorkspace.shared

      // since the new NSWorkspace function is asynchronous we have to use semaphores here
      let semaphore = DispatchSemaphore(value: 0)
      var errCode: OSStatus = 0
      
      guard let appURL = ws.urlForApplication(withBundleIdentifier: identifier) else { return 1 }
        ws.setDefaultApplication(at: appURL, toOpenURLsWithScheme: scheme) { err in
          // err is an NSError wrapped in a CocoaError
          if let err = err as? CocoaError {
            if let underlyingError = err.errorUserInfo["NSUnderlyingError"] as? NSError {
              errCode = OSStatus(clamping: underlyingError.code)
            }
          }
          semaphore.signal()
        }
      semaphore.wait()
      return errCode
    } else {
      return LSSetDefaultHandlerForURLScheme(scheme as CFString, identifier as CFString)
    }
  }
  
}