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A general description is here.
R on Windows by default uses pkg-config for linking against external libraries. This makes it easier to test R and packages with alternative toolchains (such as from Msys2, e.g. testing with LLVM and possibly with sanitizers). It also allows more significant Rtools updates within a single R minor release.
The installer scripts for Windows have been tailored to Rtools45, an update of the Rtools44 toolchain. It is based on GCC 14. The experimental support for 64-bit ARM (aarch64) CPUs is based on LLVM 19. R-devel is no longer maintained to be buildable using Rtools44 and it is advised to switch to Rtools45.