On: Finding the B-21's hangar location from the stars in its press image

[Johnny555]

Does it mean all the time it was being built, they avoided having it outside during the daytime

Aren't airplanes built entirely in a hangar? Especially secret ones?

[sophacles → Johnny555]

Depends on how you scope the word "build". If you mean literal assembly, of course. If you mean "the process of iterating engineering, testing and assembly" including prototyping etc (a common way to use the word build, especially in software circles) it makes more sense.