Your comments are just about right. You need to adjust the baud rate in the program to match that of the serial monitor, or vice versa, to see the serial output.
is an endless loop to stop the program doing anything else. It is only there for demonstration purposes. In practice you would probably make the playing of a note sequence a function and return to the main program to get more user input when it finishes.
You could play 2 notes at once in a number of ways. One way would be (yet) another array for the second note. There would need to be an entry in each array even if the second note would not always be played. An entry of zero could serve for this.
A second way would be to have double entries in one array so you would play note 0 and note 1, then 2 and 3, 4 and 5 etc. As before you would need dummy entries when no second note was to be played.
A third way would be to use a 2 dimensional array to hold the notes. Dimension 0 holds the first note and dimension 1 holds the second.
Any of these could get complicated so I suggest that you get one note at a time working first. Get my code working to print output to the serial monitor first then explore ways to use the principals to move servos. Only then experiment with multiple notes at the same time. What is the maximum number of notes needed to play at the same time ?