The Battery Disconnect Unit (BDU) contains the contactors, fuses, pre-charge circuit and current sensors. This unit sits inside/on top of the battery pack and has all of the components for monitoring, activating, and deactivating the high-voltage battery system.
Looking into the BDU of the Volvo XC40 Recharge. This is bolted on top of the battery pack, contained within an extruded aluminium base and with a thin aluminium top cover.
You can clearly see the contactors, fuse and current sensor.
The BDU for the Audi etron GT quattro is on the top and at the front of the battery pack.
The BDU is often placed so it can be accessed or removed as a self contained unit. This is because it contains the parts most likely to fail in the battery pack.