CMake library subproject

I have the following project structure.

project
    -> build
    -> sdk
        -> calculator
            -> include
                -> calculator.h
                -> calculator_p.h
            -> src
                -> calculator.cpp
                -> calculator_p.cpp
        -> interfaces
            -> icalculator.h
    -> client
        -> main.cpp
    

I want to create a library from sdk and use it in client, so client can see icalculator.h and calculator.h, but not calculator_p.h. Official cmake documentation has very poor examples, unfortunelly. I digged through a lot of examples and I have to say that I'm even more lost than in the beggining.

Should I create only one CMakeLists.txt in sdk directory or another one in each calculator and interfaces?

EDIT: This is what I came up with so far.

project/CMakeLists.txt

cmake_minimum_required(VERSION 3.13)
project(test_project)
set(CMAKE_CXX_STANDARD 17)

add_subdirectory(sdk)
add_subdirectory(client)

project/sdk/CMakeLists.txt

add_library(sdk_library SHARED
    calculator/src/calculator.cpp
    calculator/src/calculator_p.cpp)

target_include_directories(sdk_library PUBLIC interfaces/include)
target_include_directories(sdk_library PUBLIC calculator/include)

project/client/CMakeLists.txt

add_executable(client main.cpp)
target_link_libraries(client sdk_library)

I build and run from the build directory.

cmake ..
make client
./client/client