The internal nodes point to leaf nodes. The leaf nodes contain pointers to the heap (table). At least with a B+Tree, the internal nodes only point to the leaf nodes in the same B+Tree structure, and not to "data proper".
So the leaf nodes, which are often about 99% of the total, contain index tuples that point to the "data proper". The internal nodes have index tuples too, but those are only used to get to the leaf nodes.
So the leaf nodes, which are often about 99% of the total, contain index tuples that point to the "data proper". The internal nodes have index tuples too, but those are only used to get to the leaf nodes.