Most Volvos built in the last decade use an AGM main battery, because start-stop and the car's many control units draw heavily on it. On the XC90, XC60, S90 and other larger models the main battery sits in the boot under the floor, not under the bonnet. Cars with start-stop but no mild-hybrid system also carry a small 12V support battery in the engine bay. When that one fails, start-stop stops working and a battery message appears.
A Volvo watches its battery through a sensor on the negative terminal. After a new battery goes in, the monitoring system should be reset with a diagnostic tool so the car charges it as a new battery and not as the tired one it replaced. A memory saver keeps the settings alive during the change. Mild-hybrid B4, B5 and B6 models have a 48V battery too. That part is dealer work. The 12V battery is not.