From the FAQ[0]

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helloSystem: We have been in fact working with helloSystem! As some people have noticed, Release 0.2.X was basically helloSystem. (That was the second PoC. The first had been built on vanilla FreeBSD and had no GUI at all.) Under the hood, however, release 0.2.2 has a partial implementation of Cocoa, a modified compiler & linker that support frameworks, and several other additions that make it distinct. We have similar philosophies, and share technology and cooperate where it makes sense (e.g. Filer), but the project goals are quite different.

helloSystem wants to create a computer that is simple to use, open, elegant, small and fast. Older Mac OS X and Classic are an inspiration to what that might look like, but they are not explicitly trying to create an open-source Mac. In fact, they're mostly avoiding Objective-C and XML plists and other Mac technology in favor of simpler and/or more modern ways (e.g. Qt, C++, JSON).

ravynOS is explicitly trying to be compatible with Mac software at a source and eventually a binary level, without losing support for FreeBSD/X11 software, and to implement a very similar experience on the desktop and at the command line. For example, on ravynOS you can type open -a MyApp image.jpg and have image.jpg open in MyApp. You will find things in (mostly) the same directories as a Mac, like ~/Library or /System/Library/Fonts. This project is not as concerned about keeping the OS as small and simple as possible, and more concerned about making it clean, secure, performant, and compatible - implementing many of the features I use daily in macOS while skipping the lock-in and "tabletization" of the computer.

Some of the more technical differences between ravynOS and hello are:

- ravynOS uses a patched kernel with support for Mach, just like xnu, and a compiler suite that supports Frameworks and has preliminary support for Mach-O as well as

- ravynOS has a package repo with software built to "standard" paths like /System, /etc, and /usr instead of the FreeBSD repos which are built into /usr/local

- ravynOS desktop is being written from scratch with Cocoa, Mach, and OpenGL on a Wayland compositor, where hello's desktop is based on openbox, Menu and Filer (originally from LXQt) plus other lightweight services for notifications (dunst), screen color temperature (redshift) etc

- helloSystem uses a simplified .app structure whereas ravynOS uses real Bundles for .app

- helloSystem tends to use typical Unix paths and files, while ravynOS is moving towards typical Mac paths and files

- ravynOS tries to provide the same APIs as macOS

[0] https://ravynos.com/faq.html