Ask HN: How do companies decide who is a senior developer?

I have 18 years of professional experience with developing software. I have been employed with six companies through these years and had a three year stint working on my own company. I recently applied for a job, where they offered me a developer position, although they had an opening for a senior developer position. What exactly are most companies looking for a senior position? Is it mostly the way you sell yourself? Because even with my years of experience I have never put a label on myself as a "senior". I always thought that this is implied.

Every organization differs, of course, making this difficult to usefully answer.

I have always prefered seeing senior devs as capable of architecting and implementing solutions (better yet delegating parts to juniors) such that they can launch a product themselves, where a mid level dev can launch individual features but would struggle with the complexity of choosing both an auth model and a persistence layer for it (mixing well with other persistence uses) and then what the front end should look like overall, and whether to use websockets or... and a junior can fix bugs, or work on tickets/scaffolded tasks in a productive manner.

OK, here's my personal hypothesis, as I have no idea what are each company's credentials for such title, but here we go:

If, for instance, you have 10 years of professional experience in a specific tech stack, let's say front-end development, you have mastered your craft, and you deliver faster than your deadlines, my personal opinion this should sound like a senior developer's level of expertise.

I could be wrong though, but I'm talking from experience that I have discussed such topic with acquaintances of mine.

A good chunk of i is down to luck and negotiation ability, how you sell yourself to a potential employer.

In my experience, years of professional experience != Seniority

You could have been for 18years in a company where you were the senior just because you were the only one who knew how the old monolite work. You could have been for 18years in a company without knowing more than what you were just doing. This just to say that the seniority it's not something you obtain with your years but with the experiences and situations you find yourself in and especially what do you learn from them.