/diy/ - Do It Yourself

Hi everyone, I know little to nothing about electronics but I'm trying to get into it. Here I have a very simple circuit; when you press 1 (see pic) on the trace it makes the led light up and it makes component 5 play a tune. 3 and 4 are the batteries.

My questions are:
- why does it need two batteries? Was it just the manufacturers choice because they thought that for whatever reason they could save money this way or is there another reason?
-What is the blob on 2?
-what is 5 exactly? An active buzzer?
-how does it know what notes to play in the little tune? They must be hardcoded somewhere, right? Is that in the buzzer or in the circuit? could the note sequence be changed using an Arduino or something?
-is it also possible to remove the buzzer that is there right now and place my own buzzer that plays whatever note sequence I tell it to?
-assuming I had a multimeter, where would I need to place the pins to know what's going on in the circuit?

Sorry if they're dumb questions and I'm not sure if they all make sense. Thanks anyway