Windows compatibility:
* Windows platform compatibility is achieved with a slightly patched version of Codeweavers' Wine fork - virtually all of the hard work of Windows compatibility has been achieved by the Wine developers and Codeweavers and was already available open source before today.
* Interestingly, Apple is distributing this thing as a Homebrew library. Has Apple ever done this before? [1]
DirectX compatibility:
* In addition, outside of the core Wine based stuff, there's a framework called D3DMetal.framework. This is a DirectX 9-12 compatibility layer akin to DXVK used in Proton as a compatibility layer from DirectX 9-11 to Vulkan. This is what seems to be the game changer here compared to before. Before today, running DirectX on macOS was possible but lost a lot of performance and compatibility needing to go through Apple's old OpenGL support or a third party Vulkan intermediary layer in MoltenVK. This is direct (heh) first party Direct3D to Metal translation.
* Actually, it's more than "akin to DXVK". The D3DMetal.framework contains copyright attribution to DXVK as required under their MIT license. It's quite likely Apple ported a lot of DXVK to Metal. It's worth noting that DXVK itself doesn't support Direct3D 12 though, Proton uses another LGPL2.1 licensed library called VKD3D for that.
* However, D3DMetal.framework is very much not open source itself. Its license is actually very restrictive, seemingly only permitting use for game development/QA use cases. [2]
* The restrictive license seems to make it harder for someone like Valve to use this akin to how they use Proton on Linux in a sanctioned way. Apple seems intent on preventing developers from dumping their games on macOS with just their compatibility layer. It definitely won't stop hobbyists making better tools to continue run Windows games on Macs though.
* The fact that D3DMetal.framework appears to support DirectX 9 and 10 is interesting. No new commercial efforts use those anymore, so that's just there for what? Allowing homebrewers to run their 00s era Windows games?
[1]: https://github.com/apple/homebrew-apple/tree/main/Formula
[2]: It contains a license with the following language - "you are granted a limited, non-exclusive, non-transferable, personal copyright license to (i) install, internally use, and test the Apple Software for the sole purpose of developing, testing, or evaluating video games for use on Apple-branded products".