For a hard STEM degree, a 2.1 is considered about what you should get at minimum if you have done enough work, and a first is good. A third most likely not worth having and you will have to explain in interviews why you didn't do any better, most likely. A 2.2 is "okay" but won't help you much in finding work in general. That's how I have come to understand it anyway.

Not really, since it's the most-commonly awarded grade and awarded to a clear majority of students at many schools in many subjects. Usually it means that the student was in at least the top 60% of students (but probably not the top 10%) on their course.

Since our quirky grading system leaves most students' accomplishments indistinguishable, it's the reputation of the university you studied in that matters.

If you went to an 'elite' university a 2:1 would be generally taken to indicate that you worked hard, but you also had a social life as well. A 1st from an elite would not uncommonly come with the assumption that all you did is sit in your room and study for three years and that you might lack some of the softer skills.

All the above is, however, a massive generalisation.