xtool [0] came up on here a while ago [1]. I think it still requires you get a big xcode app bundle off a macos machine and drop that into xtool, since there's no other way to get the iOS library header files from Apple. The other parts of xcode just aren't used as far as I know. Other than that caveat, you're running builds in xtool's own environment on whatever platform you choose. You can even do code signing.

[0] https://xtool.sh

[1] https://news.ycombinator.com/item?id=44011515

Technically you can, legally not sure

It is.

You can build iOS apps on Omarchy now