Why are the build folders empty after running cmake command?

I am trying to follow the tutorial but I can't figure out why my build_debug and build_release are empty and nothing is compiled when I run the cmake -DCMAKE_BUILD_TYPE=Debug ../ followed by make

and I am trying to compile the following

#include <gtkmm.h>
int main(int argc, char * argv[])
{
    auto app = Gtk::Application::create("org.gtkmm.example");
    return 0;
}

as for the contents of the parent CMakeLists.txt

cmake_minimum_required(VERSION 3.26)
project(exmaple)
find_package(PkgConfig)
pkg_check_modules(GTKMM gtkmm-4.0)
add_subdirectory(src)

and the child CMakelists.txt in src

link_directories(
    ${GTKMM_LIBRARY_DIRS}  )

include_directories(
    ${GTKMM_INCLUDE_DIRS}  )

add_executable(app
    main.cpp)

target_link_libraries(app 
    ${GTKMM_LIBRARIES}  )

after running cmake -DCMAKE_BUILD_TYPE=Debug ../ both build folders are empty. The directory tree looks like this:

.
├── build_debug
├── build_release
├── CMakeCache.txt
├── CMakeFiles
│   ├── 3.26.1
│   │   ├── CMakeCCompiler.cmake
│   │   ├── CMakeCXXCompiler.cmake
│   │   ├── CMakeDetermineCompilerABI_C.bin
│   │   ├── CMakeDetermineCompilerABI_CXX.bin
│   │   ├── CMakeSystem.cmake
│   │   ├── CompilerIdC
│   │   │   ├── a.out
│   │   │   ├── CMakeCCompilerId.c
│   │   │   └── tmp
│   │   └── CompilerIdCXX
│   │       ├── a.out
│   │       ├── CMakeCXXCompilerId.cpp
│   │       └── tmp
│   ├── app.dir
│   │   ├── build.make
│   │   ├── cmake_clean.cmake
│   │   ├── compiler_depend.internal
│   │   ├── compiler_depend.make
│   │   ├── compiler_depend.ts
│   │   ├── DependInfo.cmake
│   │   ├── depend.make
│   │   ├── flags.make
│   │   ├── link.txt
│   │   ├── progress.make
│   │   └── src
│   │       ├── main.cpp.o
│   │       └── main.cpp.o.d
│   ├── cmake.check_cache
│   ├── CMakeConfigureLog.yaml
│   ├── CMakeDirectoryInformation.cmake
│   ├── Makefile2
│   ├── Makefile.cmake
│   ├── pkgRedirects
│   ├── progress.marks
│   └── TargetDirectories.txt
├── cmake_install.cmake
├── CMakeLists.txt
├── Makefile
└── src
    ├── app
    ├── CMakeFiles
    │   ├── app.dir
    │   │   ├── build.make
    │   │   ├── cmake_clean.cmake
    │   │   ├── compiler_depend.make
    │   │   ├── compiler_depend.ts
    │   │   ├── DependInfo.cmake
    │   │   ├── depend.make
    │   │   ├── flags.make
    │   │   ├── link.txt
    │   │   ├── main.cpp.o
    │   │   ├── main.cpp.o.d
    │   │   └── progress.make
    │   ├── CMakeDirectoryInformation.cmake
    │   └── progress.marks
    ├── cmake_install.cmake
    ├── CMakeLists.txt
    ├── main.cpp
    └── Makefile

15 directories, 50 files

You did an in-source build instead of an out-of-source build. You can specify where the build directory should be using the -B flag of the generate command, and the source directory using the -S flag of the generate command.

Here's the table from the docs describing what happens if you specify none, some, or all of those flags:

Command Line	Source Dir	Build Dir
cmake src	src	cwd
cmake build (existing)	loaded	build
cmake -S src	src	cwd
cmake -S src build	src	build
cmake -S src -B build	src	build
cmake -B build	cwd	build
cmake -B build src	src	build
cmake -B build -S src	src	build

Seeing that you pre-created build_debug/ and build_release/, and that you're using -DCMAKE_BUILD_TYPE=..., which is only for single-config generators, what you probably meant to do was something along the lines of the following (with the current working directory being the root directory of your project (the one containing src/)):

cmake -S . -B build_debug -DCMAKE_BUILD_TYPE=Debug ...
cmake -S . -B build_release -DCMAKE_BUILD_TYPE=Release ...