machines detect the presence of a tape cartridge by means of an infra red transistor and receiver on top of a tiny post that goes into the cartridge when the tape drops down on the spindle. if the post is mis-aligned the tape will eject. if this post is broken off - the tape will eject. if the receiver or transmitter transistor is shot - the tape will eject.
when a cartridge is inserted into the machine, the tape transport grabs the cartridge pulls it in and drops it down on the platter. a pawl opens the tape door of the cartridge and two fingers go behind the tape and pulls it out of the cartridge and wraps it around the tape head which by now is spinning. the head is tilted at an angle and reads the tape across the width at an angle. one spindle is driven with gears and belts, the other is free wheeling. for rewind this mechanism switches sides to rewind with gears and belts.