What you see and hear in the final waveform is the sum of all the instruments, the sum of all the individual sources.
All those sounds can be encoded in a single waveform. In the case of your first example (stereo sound) you have two channels (two waveforms, two signals) instead of one.
In other words, yes, it is a single sound (or two sounds in the case of stereo). It isn't different to what happens when you hear a sound live. Your ears pick up the sum of all the sounds in the room at that particular time and position. The same sum is taking place acoustically in the room and digitally or electronically in a waveform.
A 20Hz tone
and a 200Hz tone
can be summed into this single waveform, which contains both:
This also means that the sum waveform is a description of the interactions of the summed sources. We see that the new waveform has more peak amplitude than the individual waveforms.
This interaction can be destructive too.
summed with an identical waveform, but with inverted phase
will cancel each other out.
The same is happening in more musical scenarios, in everyday songs.
and bass line
can be all summed into one single waveform that contains them all.