As a pop pianist I'm trying to understand the logic in the alberti bass modulation that sounds beautiful and "right" but the notes seem way off the diatonic scale of G maj.
The key signature is G major. K.545 2nd Movt Bar 3 the alberti bass line is GECE GECE then GC#A#C# (with a C# in the RH melodic line).
My thought process would be: B♭ instead of A# makes more sense (borrowing G min?) and where does the C# come from (D♭ maybe as an GB♭D♭ looks like a diminished triad).
So why GA#C# instead of GB♭D♭?
The next bar reverts to G maj alberti base so is this a case of adding leading notes?