A formula runs once per row. During that runtime, it only has access to data within the same row. You can see a similar discussion here.
That said, I think linked records and lookups could help a lot here, hopefully negating the need for a formula. Using your example, you could set something up like this:
Fields I’ve added or edited:
Post Parent (linked record)
Post Parent ID (lookup)
Page Title Page ID Post Parent ID Post Parent
Media Kit 3 2 About
Take a look at the linked documentation above. The short of it is ‘linked records’ are reference to other rows in the base. While a lookup allows you to grab other field values from that linked record (such as the Page ID).