Cmake generator expressions

I'm trying for long time to understand the benefit of generator expressions such as $<xxx:yy> in CMake, when and how to use them. Can anybody explain it clearly with some examples. Many thank in advance

CMake does first parse the CMakeLists.txt files in your project - named "Configuration Phase" - and then it generates your build environment - named "Generation Phase".

So basically the generator expressions are for everything only the generator could know:

The name and path of target outputs (mainly when cross-compiling and in multi-configuration environments)
Or more generally any target property that the generator evaluates to mingle together the compiler/linker calls

Here are examples where I use generator expressions in my project:

Copying files next to the executable (in multi-configuration environments you can't just use variables like CMAKE_CURRENT_BINARY_DIR)

add_custom_command(
    TARGET library1 
    POST_BUILD
    COMMAND ${CMAKE_COMMAND} -E copy
        "$<TARGET_FILE:library1>"
        "$<TARGET_FILE_DIR:mainProject>/$<TARGET_FILE_NAME:library1>"
)

CMake post-build-event: copy compiled libraries

add_custom_command(
    TARGET myBinary 
    POST_BUILD
        COMMAND ${CMAKE_COMMAND} -E copy  
                     "${CMAKE_CURRENT_SOURCE_DIR}/myTest.txt" 
                     "$<TARGET_FILE_DIR:myBinary>/myTest.txt"
)

how do I add external test files to a cmake project

Differentiate e.g. DEBUG or RELEASE configurations
```
add_compile_options("$<$<CONFIG:DEBUG>:/MDd>")
```
For Cmake, can you modify the release/debug compiler flags with `add_compiler_flags()` command?

Modern way to set compiler flags in cross-platform cmake project
With the TARGET_PROPERTY generator expression you could do a lot of things e.g.
```
file(GENERATE 
    OUTPUT "includes.txt" 
    CONTENT "$<TARGET_PROPERTY:motor,INCLUDE_DIRECTORIES>\n"
)
```
CMake doesn't pick up INTERFACE_INCLUDE_DIRECTORIES of linked library