CI systems often run their workloads in virtualization (for both security and ease of uniform deployment), but sometimes the jobs themselves use a VM to either run part of the build process (such as depending on a tool distributed using Docker, which relies on such due to the host kernel not being Linux) or run some of the unit/integration tests (whether to create a clean environment or to take advantage of the hypervisor to get fast emulation of a target device, such as an Android phone or whatever). Without nested virtualization, services such as GitHub Actions (or locally hosted options; FWIW, GitHub also lets you bring your own "runners") have thereby been somewhat crippled on Apple Silicon.
Oh, that’s an excellent point. Things like WSL2 are very seamless so I don’t normally think of them as being virtualisation even though they obviously are. I might even use this myself as I need to document software development tools for Windows users.
That would fix a current blocking problem, as the lack of nested virtualisation means Docker Desktop (which runs its containers inside a Linux VM) has to run on the host and can't run inside a VM.
Correct, the simulators run regular macOS processes that are linked against a different set of userland libraries than usual. No virtualization involved.