This article by thoughtworks pretty much give a 101 introduction to story points. Story point estimation are done by individual teams with differing type of work which is also why Team A has their own velocity, likewise for Team B and Team C. Velocity shouldn’t be used as a form of comparison among teams.
Story points should not be explicitly linked to man-days of effort, as they are meant to be a purely relative estimate. Advantages include relative estimates are quicker and easier to do and to avoid over-thinking. It is also more likely to agree that story A is likely to be x times as much work as story B than getting man-days estimation from a junior engineer and senior engineer.