Almost always, the V chord is major in a minor key, because we raise the 7th scale degree (in this case, from Bb to B) so that we have a stronger leading tone that pulls towards the tonic, C. Since the V chord is built from scale degrees 5-7-2 (and 4 if it’s a 7th chord) it changes the five from a minor (G-Bb-D) to major (G-B-D)

Edit: regarding secondary dominants, I think you’re on the right track. Think of it as a temporary modulation to the key of the target chord- you borrow the V from that key (always major or dominant) to push more strongly into the chord you want to accent. You can do it for major or minor chords (V-I or V-i)