Hello, Im wondering what the "correct" technique for the left or "fretting" hand is. For example on Youtube..etc i see some people having their thumb over the top a bit during solos and what not and others strictly behind the neck. Im wondering which way i should have my left hand, arched over or strictly behind the neck.

what ever feels more comfortable. most people just have the thumb above the fret board when soloing because they don't need the reach on the thinner strings.
also depends on your hand size. i have smaller hand so i tend to keep my thumb at the back of the neck in order to reach farther frets or higher strings. Paul Gilbert or Steve Vai have these huge hands so they can reach the top strings even with their thumb on top of the neck.
The only time you should have your thumb on top of the neck is when bending or vibrato...ing All other times it should be behind, and parallel to the frets, as it gives you much better reach and makes it easier to stay relaxed.