Reusable upper stage

I'd thought the SpaceX Starship was the upper stage which could land back on Earth. But Zim is pointing to another design - by another company.

As these pressers note: the upper stage is already out of the Earth's atmosphere, mostly, so plays by different rules than a Falcon 9 or a Heavy Booster. Stoke Space inform/s us that upper-boosters have a larger "bell shaped" nozzle. Which is great for space but not so much if you are trying to land it again on Earth. That's the part I didn't know.

I am unsure how Starship accounts for all this apart from just making everything out of stainless-steel.

Other ideas I've seen (elsewhere) include reusing the tanks for additional storage-space in LEO at which point the mission does, what exactly, with the engines. Toss 'em? Sell them to other space-stations in LEO, for point-to-point transfer? Anyway there's no economy for preowned second-stage rockets in LEO, as yet; so, Stoke (like SpaceX) are working to bring them back to Earth.