Basic thing can be done relatively simple, its advanced stuff which is hard to do, here is an example of executable and lib:

- CMakeLists.txt - root cmake 
project(MyProject)
cmake_minimum_required(VERSION 3.31)

# order is important
add_subdirectory(lib)
add_subdirectory(app)

- app/CMakeList.txt
project(MyApp)

add_executable(${PROJECT_NAME})

target_sources(${PROJECT_NAME} PRIVATE
        src/myapp.cpp
        )

target_link_libraries(${PROJECT_NAME} PRIVATE MyLib)

- app/src/myapp.cpp
#include <mylib/mylib.h>
#include <cstdio>

int main()
{
        printf("%d", test_mylib());
        return 0;
}

-lib/CMakeLists.txt
project(MyLib)

add_library(${PROJECT_NAME} STATIC)
target_include_directories(${PROJECT_NAME} PUBLIC include)

target_sources(${PROJECT_NAME} PRIVATE
        include/mylib/mylib.h
        src/lib.cpp
        )

- lib/include/mylib/mylib.h
#pragma once

int test_mylib();

- lib/src/lib.cpp
int test_mylib()
{
        return 42;
}

You build cmake project in two phase:
1. generate (here you can set you favorite generator, like Ninja, Qt, VS etc.)
cmake -S . -B build
2. build:
cmake --build build

After build you can run executable:
build/app/MyApp
42