BCM is Body Control Module. ECU is Engine Control Unit. 2 different parts (logically). Both are physically PCB's (Printed Circuit Board) that physically can sit either side by side or in very different sides of the car - that's car maker decision. I can't explain it simpler than this.
ECU is usually an Electronic Control Unit. Some OME's name one of there ECU's, Engine Control Unit to make stuff more complex. Not app manufacturers have Engine Control Unit or a Body Control Module. The electronic architecture of a Volkwagen, GM, Volvo or Tesla is very different. Hardly any of the ECU's have the same name.
Or PCM (powertrain control module, since the transmission and engine typically work as one unit) which has been used by Ford (and Mazda) since ODB-2 was standardized in the 90s.
It depends on the manufacturer. Some OEMs call the Engine Control Unit the ECU while others use the general terminology where all Electronic Control Units are ECUs and the engine controller is an Electronic Control Unit named the PCM (Powertrain Control Module) or ECM (Engine Control Module). In those cases, there are some strange ECUs... ex. the steering wheel is an ECU, the stereo headunit is an ECU, etc.
