Herbie asked:

"Built on G in the key of C ?? WHY C ?
because if it was G it'd be f# right ?"

Mike proposed that we consider the 5 chord for our
"jazz" experimentation, because that is the most
frequently "extended" chord, even in non-jazz
settings (at least I think that's why he chose 5).

In the key of G, the 5 chord is D, not F#.

There is an F# in the D chord, is that what you meant?

Anyhow, the G7 chord is nearly interchangeable with
the normal (non-extended) G chord, in the key of C.

So, lot's of folks are used to that "extended" G7
sound, whether they realize it or not.

Here's a mind bender. Guess what chord is "hiding"
inside of the G7?

If you guessed B diminished (Bdim), give yourself a
gold star!

Now, what makes that surprising is that most folks,
when hearing a diminished chord, of any type, would
swear they would never want to hear that sound in a
song. Except maybe spooky movie music.

Well, folks that love "beautiful" music, anyhow.

Yet, they hear it, and love it all the time, without
realizing.

It's all about what came before, and what comes after.

HTH,
Emmit Sycamore