The graduation of specialist levels is subjective; it depends on both the company and the specialist, and even on how confidently and charismatically the specialist presents information 🙂 (Be careful with this, you can hire an incompetent one).

For me, the level of a specialist is measured in independence. Signor works as autonomously as possible, regardless of the type of task, up to the automation of work. Middle needs help making difficult and important decisions and developing scripts.

In fact, all the missing knowledge can be filled when the task appears if the specialist is experienced enough to google and apply the information.

To sum up, Senior is an experienced Middle with knowledge of related areas.
P.S. I am not writing about things like knowing the basics, tools, strategies, etc. It goes without saying.