128-bit integers aren’t part of the standard (yet)… but the point is right; platform and architecture matter. Many aspects of them are defined by the compiler implementation.

Also ABI compatibility isn’t part of the C specification. Most implementers use System V ABI compatibility guidelines. Many compilers can output code for different ABI’s calling conventions. There’s more than one! Which you use depends on how crufty the code is and on the platform you are using.

I think the big advantage of C like languages here is the plethora of implementation defined platforms. That’s a huge network effect. If you want to target the 6502? There’s a compiler for it. Some chip you’ve never heard of? Probably some fork of GCC that targets it. Want to call some library function in some foreign binary written in a completely unknown language? If you can finagle the bytes and reverse engineer the calling conventions you can do it with C.

I think it might be that C does leave so much out of the specification and up to the implementation and platform that makes it unique and useful at times.