Where is the parent/child relationship between nodes stored in the database?
The simple answer is, there is none.

Nodes in Drupal does not have a hierarchy, it's other things on top of it that allow you to do that, depending on the use case:

Menu links: You can have menu trees and each points to a node, but the nodes themself don't really about that. Some parts might look like it does, e.g. breadcrumb but that is simply based on the path.

Book module: This allows to create a book (basically a hierarchic structure of nodes), but this only makes sense for specific cases, e.g. structured documentation that has chapters and so on.

References: In many cases, nodes are just connected between each other with reference fields, which you could make look like a hierarchy.

Taxonomy: You can have Terms with a hierarchy and reference those from nodes, but again by default that doesn't give you anything hierarchy-like unless you start to combine with with path aliases or so.

=> Content in Drupal is very different from how most other sites handle it and you can't just convert it 1:1.