Hi, I'm a new member 
I love biology and recently I have been studying the electron transport chain. I'm learning it on my own, so I don't have anyone to ask questions to. The concept itself is alright, but it the end...
I was wondering: how is ATP used?
So I searched it up on google and it told me that it split it back into ADP and a Pi, and that's when ATP is referred to as being 'used'. However, if we had ADPs left as 'waste' from the usage of ATP, why don't we have an unlimited supply of ATP? (Since the production of ATP requires ADP...)

A bunch of other questions kind of branch from the main one...
1) Where does the very very first ADP come from if it is, indeed recycled back into the cellular respiration cycle?
2) If the ADP is not recycled where does it go?
3) What happens to the Oxaloacetate in the Krebs cycle? (Unrelated, but... excuse me)
4) Where does NADH come from? What exactly is it?

I'm in year 9, and this is just out of pure curiosity- please don't bombard me with a bunch of chemistry as I don't understand!! (sorry  )

Thanks in advance to everyone who answers!