I am on Verizon 33.1 on stock iOS. Haven't seen any carrier update.
I got the AT&T carrier update the day I installed iOS 12 on my iPhone 8+.
So I made sure when I set up my iPhone XS max that I went into the about screen right away to get the carrier update.
Guessing each carrier runs separate versions?
To add to the mobile reception part of this thread I do believe my iPhone Xs Max is getting slightly worse reception in my area on AT&T. I live in New Hampshire so not exactly big city living
Often times reception can be spotty in areas. Right now I wouldn’t say the reception is drastically worse than my iPhone 8 Plus but I won’t know for sure until later this week as I use it more for actual calls or cell data.