I don't know where you got that idea. OrbStack absolutely runs a Linux VM. That Linux VM then uses Linux containerization technologies (namely LXD) for each separate OrbStack 'machine' you set up, which is how you get such fast startup times for your OrbStack 'machines'.
For Docker, OrbStack does the same thing as Docker Desktop, Podman Desktop, Rancher Desktop, etc., which is set up a Linux VM running Docker and then present a native socket interface on macOS which relays everything it receives to the Docker socket inside the VM.
macOS doesn't have native capabilities for running containers, which is why the nearest thing you can get to containerd on it requires you to disable SIP so it can use a custom filesystem to emulate bind mounts/null mounts: https://darwin-containers.github.io/
If you read the PRs where the principal author of the Darwin Containers implementation is trying to upstream bits of his work, you'll see containerd comparing his approaches to others and complimenting them by calling them 'the most containerish' because real capabilities aren't there.
(I believe I've read rumors here on HN that Apple has those features internally, fwiw. But they've evidently never released them in a public copy of macOS.)
Another clue in all this is to just run uname in any of your Docker containers in OrbStack; you'll see they're Linux machines. Some operating systems have Linux syscall emulation layers (WSL1, FreeBSD's Linux emulation, Illumos' LX Zones) that could perhaps be used to run Linux containers without hardware emulation or paravirtualization in combination with some native containerization capabilities. Afaik Illumos' LX Zones is the only implementation where that's a supported, intended use case but maybe FreeBSD can do it. At any rate, macOS has never had that kind of syscall compatibility layer for Linux, either. So when you run `uname` in a 'macOS container' and see 'Linux', you can be certain that there's a VM in that stack.
PS: Aside from the fact that it's proprietary, I really do quite like OrbStack. It's the nicest-to-use implementation of something like this that I've tried, including WSL2 and Lima. The fact that it makes the VM machinery so invisible is very much to its credit from a UX perspective!
Interesting! I'd swear that in the early days of OrbStack somewhere on their website I've read they're using native MacOS frameworks without the need of Linux VM, but I can't find that anymore (they don't mention Linux VM either, but the language still differs from what I remember).
They do use native GUI frameworks rather than something like Electron, which they still mention. And maybe they also used to have something about relying on Apple's Virtualization Framework or something like that, rather than qemu as Lima used for a long time. (I think it may still be Lima's default, but not for long.)
For Docker, OrbStack does the same thing as Docker Desktop, Podman Desktop, Rancher Desktop, etc., which is set up a Linux VM running Docker and then present a native socket interface on macOS which relays everything it receives to the Docker socket inside the VM.
macOS doesn't have native capabilities for running containers, which is why the nearest thing you can get to containerd on it requires you to disable SIP so it can use a custom filesystem to emulate bind mounts/null mounts: https://darwin-containers.github.io/
If you read the PRs where the principal author of the Darwin Containers implementation is trying to upstream bits of his work, you'll see containerd comparing his approaches to others and complimenting them by calling them 'the most containerish' because real capabilities aren't there.
(I believe I've read rumors here on HN that Apple has those features internally, fwiw. But they've evidently never released them in a public copy of macOS.)
Another clue in all this is to just run uname in any of your Docker containers in OrbStack; you'll see they're Linux machines. Some operating systems have Linux syscall emulation layers (WSL1, FreeBSD's Linux emulation, Illumos' LX Zones) that could perhaps be used to run Linux containers without hardware emulation or paravirtualization in combination with some native containerization capabilities. Afaik Illumos' LX Zones is the only implementation where that's a supported, intended use case but maybe FreeBSD can do it. At any rate, macOS has never had that kind of syscall compatibility layer for Linux, either. So when you run `uname` in a 'macOS container' and see 'Linux', you can be certain that there's a VM in that stack.
PS: Aside from the fact that it's proprietary, I really do quite like OrbStack. It's the nicest-to-use implementation of something like this that I've tried, including WSL2 and Lima. The fact that it makes the VM machinery so invisible is very much to its credit from a UX perspective!