ECM controls the engine.
WARNING: This page is about a different car, the 2011 Nissan Juke. However, it is still accessible from the selected car via links, so may be relevant.
When power supply position is turned ON, BCM starts communication with ECM and performs the ID verification between BCM and ECM.
If the verification result is OK, the engine can start. If the verification result is NG, the engine can not start.
When ECM restarts engine by the Stop/Start System (CVT models), ECM transmits the control signals to IPDM E/R and BCM to crank engine.