is the following a correct description ?

• You described an algorithm with three conditionals and a loop. The algorithm only has a loop.
• You described an algorithm with two recursive calls. The algorithm only has one.
• You described an algorithm with an infinite loop.
• You described an algorithm that adds to the callee's array after the callee has returned. That's impossible.

The algorithm is:

• Given the root of a subtree $self,
  • $self is a node of subtree $self.
  • For every child of $self,
    • The nodes in the subtree rooted at that child is a node of subtree $self.
  • Return the nodes of subtree $self.

i had the impression that if you push an ARRAY into an ARRAY, the ARRAYS are preserved (not merged).

It sounds like you're expecting an array to be able to contain another array. It can't, at least not directly. Array elements are scalars.