Teach me keyboard and guitar

Hi anyone know the theory behind how to press the slash chords of the guitar? Can teach me so that i will know how to play more accurate chords?

Secondly, regardling the keyboard, how to play the inbetween chords? Is it for example in the key of C, the inbetween chords can be a varies of C, Dm, Em, F, G, Am and B for the right hand or for both hands when they didn't mention any chords to play?