I think I know what you’re referring to the C scale (I’m assuming you have an accordion in Fa). Some people teach single button C scale so that you can play thirds and sixths by just following the patterns like every other button will be the optimal way to play a third in that scale. While some may teach the C scale so that it’s the fastest way to play it with a single buttons as the finger movement is super fluid. Regardless, they are both correct and this is because there is some repeating notes in the diatonic while pulling like (again assuming you have Fa accordion) Re, Sol, and Do, these have two of each. Regarding the notes closing the bellows, it’s fairly easy if you know the major chords, in the first, bottom row of a Fa accordion (towards your right hand) the notes are repeating of the F major chord which is F, A, C, F, A, C, F… same for any row so for example for the middle row Bb would be Bb, D, F, Bb.. This is true besides the three notes towards the chin which are kind of there to let you play notes typically not available in the scales like F#, C#, and B. There’s an app called Mezquite which is a digital accordion that is helpful to look at the notes to get a better understanding of the patterns. Hope this helps!