TLDR here is that Julia is better described as Just Ahead of Time (JAOT) instead of JIT. Julia isn't using a normal tracing JIT where things start running interpreted and then get replaced. When Julia first runs into a new function (or function being called with new argument types), it will statically compile (at runtime) code for that function being called with those argument types. When it does this, it will use the type information of the types the function has been called with, to do all sorts of optimization (de-virtualization, inlining, etc).

Once this method is compiled, it will be used whenever the same function is called with the same argument types.