BMW puts the battery in the boot on most models to balance the car's weight. It sits under the floor or behind a side panel, with a vent tube and a sensor on the negative terminal called the IBS. That sensor tracks every amp going in and out. Most BMWs of the last fifteen years, start-stop or not, were built with an AGM battery, and it should be replaced with the same type and amp-hour rating.
Registration is the step that separates a proper BMW battery change from a quick swap. The car's computer keeps a record of the battery's age and charges an old one harder. If it is not told that a new battery has gone in, it overcharges it and may keep switching off comfort functions to save power. If the capacity or type changes, the car is also recoded. A memory saver is used, and stored low-voltage faults are cleared.