It's pretty arbitrary, and depends also on what you negotiate when getting hired. I've seen people that got hired as seniors that performed just as well/poorly/

It's pretty arbitrary, and depends also on what you negotiate when getting hired. I've seen people that got hired as seniors that performed just as well/poorly/... (depending on your perspective) as mid-level folks. I've also seen the title of "senior" in one organization being wildly different to a "senior" in another organization within the same company. For example, for some reason, the "API dev team" had insanely high bar whereas the "submodule X team" had seniors left and right, quality-wise nowhere near seniors from the "API dev team". This all has led me to believe getting promoted is simply about renegotiating your salary with the company, and has very little to do with your actual skill. It has more to do with who's your direct manager, and what is your relationship with him/her. Personal and anecdotal experience only of course.