After the HR screen, you get the Code Pair interview. These consisted of some engineer who was clearly working on something else, not paying attention to me, or giving any feedback when I vocalized my process toward a solution.

At the start, there was even code from the last interview still in the IDE. I started asking questions about if I had to implement all these functions scattered around, and he then went and deleted it once he actually looked at the screen.

This shows how little effort they give during these interviews. Why not just send an online assessment if the engineer on the "coding pair" is not even paying attention the entire time?

It's hard to tell what exactly they are looking for since they don't give you any feedback or even talk to you during the interview.

Convert a string of an expression to perform the operations and compute the result.