Builds can't find the projects qml modules

The attached demo code runs fine on Linux here, but fails for everything else I tried.

Android - in two places. The build process itself says

qmldir file not found at "/home/me/tmp/demo/ui"

twice.

Starting the application bails out on Android with

W libdemo_x86_64.so: QQmlApplicationEngine failed to load component
W libdemo_x86_64.so: qrc:/demo/main.qml:10:5: Foo is not a type

.

Building for iOS stops with:

clang: error no such file or directory: '/User/<snip>/ui/Debug/libui.a'

Building for macOS, gives a linker warning:

<snip>/usr/bin/ranlib: file: ui/libuiplugin.a(mocs_compilation.cpp.o) has no symbols

Webassembly compilations doesn't - as Linux - have any complaints while building.

Webassembly and macOS run into

QQmlApplicationEngine failed to load component
qrc:/demo/main.qml:10:5: Foo is not a type

when trying to start.

Originally, I just wanted to make the android problem visible, but that leaves me behind a bit confused. I'm using the same mechanism in a bigger application, and at least macOS is known to work. Either I'm missing something important, or somewhere else is missing something important