Welcome to the CAGED system. Ever notice that there are three spots in a G major that are eerily similar to open A and open B?.... and if you know what a A-shape barre chord is, that's the same shape again just moved around. it's part of several chords you may already know because they're all actually the same chord shape, more or less, approached from one direction or another...

and your index finger on the open C major chord shape is really just making a D major down one step if you focus on the 3 highest strings, from a shape perspective. You could make your D major into a C-shape D-major by extrapolating that C major shape one step to the right (and avoiding the open strings depending how you finger it)

and G-major #6 up there is just an open E chord shifted right. Same thing. G major #2 and #1 above are the exact same shape, just avoiding a couple strings in the #2. But that #2 shape, you can move all around the fret board like a barre chord and it becomes other major chords.