In what folder are Child Theme located in DT?

Hi,
I just upgraded from DP 7.1.4 to DT 6.5.6. We then performed several modifications of the new DT.

Today we updated our DT to 6.5.7 but lost several modifications.

It seems that the changes we did were in the parent theme.

As far we understand the best practice in the old DP 7.1.4 was to modify child themes, not the parent theme. Any tutorial on this topic, as to the responsive DT theme, will be appreciated.

1. In what folder is the parent theme located?
2. Does the DT theme comes with a default child theme? If yes, where its located.
3. In what folder are we supposed to ftp/upload child themes?
4. What files of child themes we should not modify, if any?

We are still learning the new framework, so please be patient.

Thanks!