When building a simple library project with "install" target:

CMakeLists.txt

cmake_minimum_required(VERSION 3.10)

project(Test)

add_library(Test STATIC test.cpp)

install(TARGETS Test DESTINATION "${PROJECT_BINARY_DIR}/expexted_install_dir")

test.cpp

class Test{

};

...then the binaries are not copied because the build type is changed to "Debug-iphoneos" instead of a plain "Debug" but several CMake generated files don't support that name:

• cmake_install.cmake

  "${CMAKE_INSTALL_CONFIG_NAME}" MATCHES "^([Dd][Ee][Bb][Uu][Gg])$"

• Debug-iphoneos/install.build/Script-93426DA6A4AC2C2AC9CE883C.sh

  /Applications/CMake.app/Contents/bin/cmake -DBUILD_TYPE=$CONFIGURATION$EFFECTIVE_PLATFORM_NAME -P cmake_install.cmake
  

CMake by default handles EFFECTIVE_PLATFORM_NAME correctly because building the project for iOS without Qt works correctly and the install script is generated differently where DBUILD_TYPE and DEFFECTIVE_PLATFORM_NAME are separated:

• Debug-iphoneos/install.build/Script-93426DA6A4AC2C2AC9CE883C.sh

  /Applications/CMake.app/Contents/bin/cmake -DBUILD_TYPE=$CONFIGURATION -DEFFECTIVE_PLATFORM_NAME=$EFFECTIVE_PLATFORM_NAME -P cmake_install.cmake
  

So it must be in the way Qt configures CMake and I narrowed it down to the following argument. It is caused by CMAKE_TOOLCHAIN_FILE

CMAKE_TOOLCHAIN_FILE Qt/6.1.2/ios/lib/cmake/Qt6/qt.toolchain.cmake

...but could not go deeper anymore.