That is a big question. There are tons of flavors of Ebikes: hub motor, friction drive, mid motor, rear drive. Hub motors have the motor in one of the wheels, friction drive mounts the motor so as to drive the tire through a roller, mid motor drives thru the chain at the front sprocket, rear drive has the motor next to the rear wheel driving with a chain and sprocket.

Then their is the controller: Assist (pedal torque sensor), assist with throttle override, throttle alone.

Gotta have a battery: Lead acid, Lithium (lots of different chemistries), nickel metal hydride, hybrid drive (gas powered generator).

I have a bike I will soon be powering with a RC type airplane motor.

It all depends on what floats your boat.