An empty AppKit app without using Storyboards

AppDelegate.swift

import Cocoa

class AppDelegate: NSObject, NSApplicationDelegate {

    var window: NSWindow!

    func applicationDidFinishLaunching(_ aNotification: Notification) {
        window = NSWindow(
            contentRect: NSRect(x: 0, y: 0, width: 800, height: 600),
            styleMask: [.titled, .closable],
            backing: .buffered,
            defer: false
        )
        window.contentViewController = ViewController()
        window.makeKeyAndOrderFront(nil)
    }
}

main.swift

import AppKit

let app = NSApplication.shared
let delegate = AppDelegate()
app.delegate = delegate

_ = NSApplicationMain(CommandLine.argc, CommandLine.unsafeArgv)

ViewController.swift

import Cocoa
import SwiftUI

class ViewController: NSViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        // If you are using SwiftUI:
        let viewController = NSHostingController(rootView: YourSwiftUIView())
        self.view = viewController.view
        self.view.frame = .init(x: 0, y: 0, width: 800.0, height: 600.0)
        viewController.sceneBridgingOptions = .all
        // If not, you can use your own NSViewController
    }

}