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Hey @MarkI ;
The simple answer to your question, “please let me know why the “2” is on the “D” string and the “0” is on the “A” string” is “they are the correct notes”, or the notes that sound good.
And why do those notes sound good? Simple, they are chord tones. The open 5th string is an A note and the 2nd fret, 4th string is an E note, and they are the 1st and the 5th of the A major chord.
Same goes for the open 4th string and 2nd fret, 3rd string; it’s the root and 5th of the D chord; and the open 6th string and 2nd fret, 5th string are the root and 5th of the E chord.
As time goes by, you will learn more about these connections. You may look closer to understand, or you may choose to never understand. It doesn’t matter if you know what you are playing, as long as you learn what sounds good. It’s amazing how many famous guitar players didn’t know music theory or what they were playing. They didn’t care.
If you want an in depth, intellectual explanation, I can provide it. But it won’t make you a better guitar player and it won’t help you create original songs at this stage. But if you have that kind of mind, like me and many others, I’d be happy to explain more. Just ask.
MG 😀