What information can a third-party SDK obtain from applications using the SDK?

  java, question

Suppose I have developed an SDK on client acceleration. If QQ uses this SDK, what data can the SDK obtain from QQ? Can I access the data stored in QQ’s internal space?

As long as the APP that uses this SDK gives you enough permissions, you can basically get it.
For example:
The owner of a house gave you the key to the door to live in, but there are several rooms in which you don’t have the key, so you can’t get in and don’t know what’s inside. Of course, you can choose to break in or use other methods.