Follow this advice: https://github.com/JRASoftware/cpp23-import-std-guide

cmake_minimum_required(VERSION 3.31)

project(
    compile_with_module_std
    VERSION 0.0.1
    LANGUAGES CXX)

# hack #1: temporary hack for GCC g++16.0 HEAD https://www.equation.com gcc-15-20250420-64.exe
# to remove when g++ & CMake know about module std
add_library(module_std)

target_sources(module_std
    PUBLIC
        FILE_SET CXX_MODULES FILES
            gnu/std.cc # copied from MinGW\include\c++\16.0.0\bits\std.cc
)
target_compile_features(module_std PUBLIC cxx_std_26)
# also to remove (see below):
# target_link_libraries(...XYZ... PRIVATE module_std)

# hack #2: advanced features like std::print(), std::stacktrace, etc in g++14, g++15, and g++16 HEAD:
# target_link_libraries(...XYZ... PRIVATE stdc++exp)

add_library(my_module)

target_sources(my_module
    PUBLIC
        FILE_SET CXX_MODULES FILES
            src/my_module.cpp 
)
target_compile_features(my_module PUBLIC cxx_std_26)
target_link_libraries(my_module PRIVATE module_std) # hack #1

add_executable(demo src/demo.cpp)
target_compile_features(demo PUBLIC cxx_std_26)
target_link_libraries(demo PRIVATE my_module)
target_link_libraries(demo PRIVATE module_std) # hack #1
target_link_libraries(demo PRIVATE stdc++exp)  # hack #2

Modular src/my_module.cpp

export module my_first_module;
import std;
 
export namespace something
{
 
std::string hello(std::string name)
{
    return "Hello, " + name + '!';
}   
 
} // end namespace something

used in demo.cpp:

import std;
import my_first_module;
 
int main()
{
    std::println("{}", something::hello("modules"));
}