How is the capacity variable? 2500-6000. Is it a sliding stage thing like 9:30 or just closing sections off?

I believe its 6k, 3500 just covers the floor, add the tiers and the all the standing areas in the open arena style.

There can be significantly more standing room on floor 1 in front of the stage depending on the performance. If there was an orchestra playing, it would likely be a different capacity with the venue placing all their chairs out on the floor, as opposed to if it were a Lil Uzi Vert (a rapper) concert, the capacity would likely be more because there would be no seating in front of the stage, just general admission standing room.

It's based on seating. When they take out the seats (except for the ones in the balcony which are not removed) for events like rock concerts, they can fit more people.