You can use alternative toolchains like like cctools-port[1] and xcbuild[2]. See, for example, this article [3] on how to build Swift UI apps for iOS using Linux.

[1] https://github.com/tpoechtrager/cctools-port

[2] https://github.com/apple-cross-toolchain/xcbuild

[3] https://thi.im/posts/cross-compiling-for-ios-part-1-build-sw...