C++ doesn't understand Objective-C classes, you have to use Objective-C++ (and rename the files to .mm)

Swift ABI got stable in 10.14.4, this means that those libraries are now bundled with macOS. Probably you set the deployment target to a version older than 10.14.4, so Xcode bundles the Swift libraries, but only for Intel because the first macOS version for arm was 11, and it already includes those libraries.

As far as I know OpenGL is still deprecated, same as before. It hasn't been removed.

Because it's an private class of the NSAttributeString class cluster, and there shouldn't be any difference in the public interface.

Yes, code sign first then notarise. There are some useful topics here on this forum too: https://developer.apple.com/forums/thread/707080 https://developer.apple.com/forums/thread/707377

The way setCanDrawConcurrently works is that AppKit calls your drawRect method in another thread. And that's it, it doesn't mean you can safely draw things on yet another thread different from the calling thread.

That's the NSNotFound constant..

That seems unrelated to NSView setCanDrawConcurrently property. Either try to use setCanDrawConcurrently directly, it finally works on 10.14 and later, or do what was recommended in that stackoverflow answer.

The app will be run from a randomised path until the user manually move it: https://krypted.com/mac-security/app-translocation-services-os-x-10-12/ (I don't know the URL of the current documentation).

Where did you read it? Actually before 10.14 NSView setCanDrawConcurrently was broken and it run everything on the main-thread.

The path is correct, only sandboxed apps use the Containers folder.

It would be easier to just bundle a copy of those libs inside each app. Then you won't need a .pkg to install them, or a deinstaller to remove them. Anyway, if the libraries you replace in your /Libraries/MyAppLibs/ folder are properly signed, there shouldn't be any issue. Each .app bundle or library outside an .app bundle is a separate entity. Changing one entity won't affect the others, just make sure you keep using the same signing identity. Changing something inside a .app bundle will break its signature, macOS doesn't check it every time, but the next time it will (and it will surely will) the app won't run anymore.

I remember reading somewhere that kext are not supported on macOS VM on M1. But I can't find the source of this information or where I read it.

Sign and notarise it using your Developer ID certificate and then distribute it in the way you want: https://developer.apple.com/developer-id/

Yes, FFmpeg hwaccel can use a VideoToolbox hardware decoder, and hardware encoders are supported too. Check the FFmpeg documentation.