Since GP has replied, I'll now add: I think of the divide as "extroverts gain energy from group social interactions; introverts lose energy during group social interactions".

IMO, it's not shy vs open, as my wife is on the shy side but clearly extroverted, while I'm more visibly/apparently comfortable but I find it exhausting to be in groups for a long period of time.

I don't care to be in the center of attention I guess and I'm very comfortable with silence.

When I socialize with friends typically 1h is enough.

I don't "collect" friends. I only get close to people when it makes sense. I am not socially inept however, just selective