What is this metal piece on top of my threshold called? I'm asking because I am looking to purchase the same one for another exterior door in my house.
It's a 'threshold bar' or strip, 'threshold transition bar/strip', 'joint strip' or 'joint trim'.
Any big box DIY store will stock many types, in many finishes.
If you're not sure which type you need - they depend on what material is on each side & the relative heights - then take that picture & one of your proposed new site to the store with you, & ask.
What you appear to have in your photo are two of different types, rather than a single transition strip.
They are transition strips and, as Tetsujin mentioned, there are a number of different types. Most of the wood ones we looked at when doing some flooring did end up combining one or more pieces with a base and the number and selection depends on the height differential, profile, and surfaces on either side of the transition.
Since that one appears to have some insulation, it may be filed near the doorway/threshold sections of your local home improvement store instead of the flooring section.