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
For the runtime, there is not much dark magic. Each type that has an `extend T::Sig` has `method_added` hooks registered, which notifies Sorbet runtime whenever a method is defined on the type. When that `method_added` hook is called, Sorbet runtime uses the sig flag that you mention to associate the `sig` with the method definition that follows it.
