Data is held in the PRSUBPROJECT and PRTASK tables
See the "Projects" entity reference diagram in the Technical Reference Guide
To get the full "tree" you will need to make recursive joins (On Oracle this is the CONNECT BY syntax in SQL)
As has also been mentioned the "hierarchies" tables also give a view of that data.
Note that a project may be the child project of multiple parents (i.e. A is a parent of B, C is also a parent of B, D is also a parent of B), but the application will prevent a circular reference from happening (so A is a parent of B and B is a parent of C and C is a parent of A can not happen)