they don't really have "source code", they're more baked into the compiler
#@ function source code?
haughty tendon
someone can help you find relevant compiler code for any builtins you're interested in particular
visual tendon
i think this might be the most relevant code for @mod: https://github.com/ziglang/zig/blob/6415ff29d717ad10cd9e2bdae9e77677f8e1f8fa/src/codegen/llvm.zig#L8648-L8682
(if i understand correctly, it's emitting LLVM bitcode)