Questions about the stuff in the hell on earth starter pack

How do I make the flying sky box jets that appear in the second episode?

How do I make the realistic Nazi's?

I am using gzdoom builder

Flying sky box jets? I'm not sure what you mean, but it sounds cool!

As for creating new enemies (presuming you don't deem the Wolfenstein Nazis "realistic"!) then it's not GZDB you need, it's DECORATE, which is a Doom programing language for defining new stuff. Search for tutorials, there are plenty that'll get you started. If you know coding in other languagesyou'll pick it up pretty quickly.

I too would like to know how to make those jets. I would also love to know if something like that, even if greatly simplified, was possible in vanilla or at least boom.

I thought those jets were actors that went on a specific path defined in DECORATE (and maybe some ACS) when the player did something specific like stepping over a trigger line.

The answer to your question would be to look inside the WAD's DECORATE lumps and see how Mark did it. 

It's probably a projectile actor or something fired from mapspots in a skybox.

The realistic Nazi's are the grey ones on the secret Wolfenstein map when you use Brutal doom/project brutality