Okay so I am very confused on second dominant chords, someone described it as using that chord contains a key that’s not in the scale but can be used and it’s called a second dominant chords and will also want to take you back to the tonal key of when it started.

So for example, C major has C E G the secondary dominant is 5 steps towards the scale and it will be G. I know it’s a 7th chord so will it be: G B D and F#???? Or is it G B D and F??

Because G major contains F# and we use it to make a 7th chord but what about G B D F to me GBDF sound better than F# but what I don’t understand is how do me make a second dominant chord?