there is also big bunch of `*-mingw` (ie windows-only) packages in AUR, but for some reason i saw the AUR moderators hunting only for non-x86-64, but completely ignoring those - so indeed i think some formal clarifications would be useful

The mingw packages are used by people that want to windows programs on a linux system (cross-compiling) .

See https://wiki.archlinux.org/title/Cross-compiling_tools_package_guidelines and https://wiki.archlinux.org/title/MinGW_package_guidelines for info on how they work.