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A general description is here.
A C23 compiler (if available) is now selected by default for compilation of R and packages. R builds can opt out _via_ the ‘configure’ flag ‘--without-C23’, unless the specified or default (usually ‘gcc’) compiler defaults to C23: ‘gcc’ 15 does.
A C23 compiler is known to be selected with ‘gcc’ 13-15, LLVM ‘clang’ 18-20 (and 15 should), Apple ‘clang’ 15-16 and Intel 2024.2 (and 2022.2 should).
Current binary distributions on macOS use Apple ‘clang’ 14 and so do not use C23.
Packages are now installed using C23 where supported by the OS and R build.
Packages using R's compiler settings can ask *not* to use C23 _via_ including ‘USE_C17’ in ‘SystemRequirements’ or can be installed by ‘R CMD INSTALL --use-C17’. (Some packages ignore these settings in their ‘configure’ script or when compiling in sub-directories of ‘src’, as will those using a ‘src/Makefile’.)