Adverb position question

[J_Caesar]

1. I definitely didn't do it.
2. She is really a good girl.
3. She is a really good girl.

My questions are:
1. I thought definitely (the adverb) should be placed between didn't (the auxiliary verb) and do (the main verb).
2. Which word does really (the adverb) modifies?
3. The adverb modifies good (the adjective). Does this sentence have difference meaning to sentence 2?

Thank you very much for reading my questions :D.

[Soup]

1a. I definitely didn't do it.
1b. I didn't definitely do it.

2a. She is really a good girl.
2b. She really is a good girl.
In that context, really is synonymouos with truly.

3. She is a really good girl.
In that context, really is synonymous with very.

2. and 3. express different meanings.

[J_Caesar]

Am I correct to say that:

1a. definitely modifies the whole sentence "I didn't do it"
2a. 2b. Similar to 1a.
3a. really modifies an adjective as opposed to a whole sentence.

[Soup]

[1] 1a. The adverb definitely modifies the verb phrase "didn't do it".

[2] They're not comparable in terms of position; didn't is an auxiliary verb, whereas is is a linking verb.

[3] Yes, 3a. really modifies an adjective as opposed to a whole sentence.

_________________
Correction
Am I correct in saying that ...

[J_Caesar]

Thank you very much. It is clearer to me now.

[Soup]

You're most welcome.