GCC is a monolithic design architecture, while llvm is more modular.
LLVM has fix the m*n problem of the compiler design to m+n to know you can visit to the blog
https://www.compilersutra.com/docs/llvm/llvm_basic/Why_What_Is_LLVM
While GCC is still used in the legacy software.
To know more about the difference between gcc and llvm, please visit