"Where" does back EMF appear in motor?

The back EMF is generated in the wire that makes up the coils of the motor. When a wire is swept sideways thru a magnetic field, a voltage is generated along the length of the wire. Spin the motor with just a voltmeter connected, and you will see it make a voltage.

So yes, the resistance and the back EMF are actually distributed along the wire in the coil. There are lots (infinity, actually) of little resistances in series that each get a little voltage in series with them when the motor turns.

Viewed electrically from the outside, this can't be distinguished from a lumped resistance in series with a lumped voltage source. Since this is simpler to draw, think about, and analyze, that's how motors are usually shown.

You can simplify things by removing the battery from that diagram, and turning the motor by hand, as a generator.

As the armature turns, the wires cut through the magnetic field, and a voltage is generated in them.

This voltage is the same whether the armature is turned as a generator, or as a motor. When it's turned as a motor, this voltage is called the 'back EMF'.

Think of it like a generator; you spin the rotor and an output voltage is produced. This voltage is in series with the rotor coil. It makes no difference if instead of manually spinning the motor you apply dc to turn it.