One paragraph, many questions!
Mixdown is accomplished by switching your channel strips to monitor tape playback instead of the mic inputs you used while tracking.
You connect the 688 to an external amp and speakers to hear your results and feed that output to a 2 track recorder to create a final stereo mix. CD recorders and computers are popular choices.
Effects are added after the fact by plugging in an effects unit that can produce reverb. It gets hooked up in the effects loop connections where you have effects out to feed the verb and effects receive to return the verbs output back to the stereo buss fader and output jacks that feed the two track external recorder.
As for infinite channels; you don't have that. You have a total of 8 tracks to fill up and once you have recorded 8 tracks, you have run out of room and must bounce tracks or become more creative in how you use the tracks you have.
As an example, a backing vocal part that only comes in and out of the song during the chorus of a tune can be re-used in the silent parts for other instruments like a keyboard part or a guitar solo.
You have to be careful that these parts don't overlap and that you punch in and out at the appropriate times.
Doing that carefully and creatively, you can record more then 8 parts on the 8 tracks the tape recorder offers you.
Cheers!
