Chord tones come from the scale. In this case the V7 tells us it's the dominant scale. In Cmaj, the dominant scale starts on G. You can call it mixolydian if you want to sound techy.
G A B C D E F G A B C D E
1 2 3 4 5 6 7 - 9 - 11 - 13
1,3,5 and 7 are the chord tones.
The numbers above 8 are called extensions ( 9, 11, and 13). They are added to the chord to give it colouring. Eg G7add9 (which is usually just written G9) is 1 3 5 7 9 ( G B D F A).
It's confusing using 9 instead of 2 since they are the same note. The reason is the numbers below 8 are usually suspensions ( 2, 4 and 6 ). These are used to replace certain chord tone above. eg. G7sus2 will have the 2 note (A) instead of the 3 note (B) so it's G A D F.
8, 10 and 12 are never used.
You can make any of the extensions sharp or flat. eg. G7#9 This means a G7 chord with a sharp 9th added.
If you just want to specify a G7 with some sharp or flat extensions you write G7alt. It's up to the player to decide which ones to play. So you could add b9,#9,b11,#11,b13,#13.
A flat 11th is the same as a major 3rd. It's a bit strange to write it like that. Perhaps they are trying to get the dissonance of a #9 (minor 3rd) against the b11 (major 3rd). Guitarist might know that from the hendrix chord; E7#9.
A easy sub you can do when you see a G7alt chord is to a C#7 instead.
For a G7 chord you have G B D F and the #9, #11, #13 will be A#, C# and E#. The b9 will be G#.
C#7 has C# E# G# B# so it covers most of the extensions nicely and it has lots of tension that will be resolved when you move to the tonic chord Cmaj.