“OrbStack uses a lightweight Linux virtual machine with a shared kernel to minimize overhead and save resources, similar to WSL 2 (Windows Subsystem for Linux). This flexible architecture comes with many advantages such as high efficiency, low resource usage, seamless integration with macOS, and more.”
Seems they're maintaining an unpublished fork of the Linux kernel "linux-macvirt" to achieve this. Apparently you have to contact them directly for a copy of the code[1]
https://docs.orbstack.dev/architecture