Who is a Sr Software Engineer?

If you are referring to the title, it depends on the organization. I have seen people with 2 years of experience with "senior" titles. Titles have some meaning in the context of an organization, but there is no standard across different organizations.

If you are referring to the role, it depends on the experience. Time in the industry is part of it, but not the whole story. A senior front-end developer might be a very junior backend developer. And it gets even more nuanced. A senior front-end developer who has worked on Ruby on Rails for 10 years might be a very junior front-end Spring developer.

So.. seniority has to do with time in the industry, and experience in a given area.

In practice, you are a senior developer when everyone else comes to you to ask questions. You could be very senior in one team, and very junior in another team. It is really all relative.

In the quest of understanding when and how a software engineer becomes a Senior.