@flo I think the logic is the numbering is for above ground. 1st, 2nd, 3rd, etc are floors where people actually live, or in an office building where they work. That’s why basement isn’t 1.
It varies around the country probably how the building is numbered. Even within one city it can vary. NYC has a lot of high rises and it varies a little from building to building. That’s why the star can be helpful.
Many buildings have 2 “ground” levels. Growing up the building I lived in the basement had an entrance/exit to the outside. My sister’s building before this one also had an exit out on two different floors, and it wasn’t like you had to walk up stairs to hit the ground. The ground outside was at one level in the front of the building and a different level at the back. The building also had a basement. I don’t remember her elevator.
When I lived in Memphis I remember going to the metal museum and the person at the front who took my money told me what I would see on the lower level and the upper level. I came across stairs to the basement when I first started walking through, so I thought I’d start there on the “lower level” but as I walked down I quickly realized it was just a basement. When I asked, she said a lot of people interpret lower level that way, but she meant the main level we were on, and the upper level was a floor above us. If I’m standing on the main level or lobby level, I wouldn’t call it the lower level in conversation. Maybe the elevator (I didn’t use I) had the floors designated as LL and UL, and it wasn’t just how she says it. I don’t know.