NoMa is the old New York Avenue station which was built fairly recently. They just rearranged some tracks and slapped a platform on it. It was an easy build.

As for above ground vs underground--I'm going to hazard a guess that they'd go above ground as much as possible, since any tunneling process (boring or cut and cover) is going to be hugely disruptive and expensive. (Hell, I remember the mayhem that was the Petworth station construction job, and I didn't even live here at the time.)

For example, the Purple line in MD is hung up for another 7 months because of...utilities. The power etc utilities didn't get moved in time, and that's an entirely above ground line. Imagine if they were trying to tunnel?

Not to mention other tunnels they'd have to go around.

https://www.washingtontunnels.com/