Sorbet has its own written-in-C++ parser that does the actual parsing. At runtime, the sig call basically sets a flag for the next method that is defined which hooks into it to validate that the parameters passed in are as defined, and that the return value is as expected. I believe they're delving into the dark magic in the interpreter directly, the docs are here: https://sorbet.org/docs/runtime

The actual signature checking is complicated, but associating the sig call with the method is pretty simple in Ruby — you just hook `method_added`. (Of course the actual implementation in something like Sorbet is a lot more sophisticated than just `def method_added`, but that's the basics of how you make a method call alter the next method definition.)