Does Orisa's "Halt!" activate when it hits someone?
"Halt!" activates when the right mouse button is pressed again or it hits a "surface"
https://overwatch.gamepedia.com/Orisa#cite_note-3
What about if it hits an enemy? Say I am firing halt to a group of enemies without bothering to press the right button again. Will it work?
No, Halt! will pass right through enemies and barriers, and only automatically detonate against solid surfaces.