My question is - where is B Floor - is it a trek for arriving passengers?(who intend to leave via AOT Taxi)

This is the bottom-most floor, the basement, follow the signs for the Airport Rail Link. You'll arrive on level 2 (Arriving Immigration, baggage carousels, Customs, AoT Limo(), so you'll have to go down two floors to pick up your SIM, then back up two floors to get your AoT car. It can be a trek - a combination of travelators and/or elevators are available, all to save 150 baht. You could buy the same SIM from the DTAC kiosk (staffed, and they have an ATM-like machine) for 299 baht right on level 2 near door #7 .