It may be helpful to read Standard C++ Modules — Clang 19.0.0git documentation and https://libcxx.llvm.org/Modules.html to get a feeling for this.
We only install source files and hope the build systems to deal the building of std modules actually.
So the reason why it says “module std not find” is that the std module is not built. An example to build std module in cmake can be found in Compiler Explorer. In the future, the cmake should be able to do that automatically.
Also -fmodules is for clang modules, it is not for C++20 modules.