Alternative toolchains on Windows

R and R packages on Windows, if they include native code, are built using compiler toolchain and libraries from Rtools. There is always a specific version of Rtools for a given version of R. Rtools44 is used for R 4.4.x, and hence R 4.4.x and packages for that version of R are always built with GCC 13.3 for x86_64 systems and with LLVM 17 for aarch64 systems. This is the case at least since Rtools42 (R 4.