Weight isn’t anything like a critical as one would assume. Weight distribution however is a different matter and this is where geometry, specifically the ratio of the front centre measurement to the chain stay length really comes into play. Battery placement is important too, and smaller batteries give an edge in the handling stakes while robbing the bike of range.
The two most common types of hub motors used in electric bicycles are brushed and brushless. Many configurations are available, varying in cost and complexity; direct-drive and geared motor units are both used. An electric power-assist system may be added to almost any pedal cycle using chain drive, belt drive, hub motors or friction drive. BLDC hub motors are a common modern design. The motor is built into the wheel hub itself, and the stator fixed solidly to the axle, and the magnets attached to and rotating with the wheel. The bicycle wheel hub is the motor. The power levels of motors used are influenced by available legal categories and are often, but not always limited to under 750 watts.