The first measure has G-B-E-B, i.e. E minor tonic chord, first inversion. Makes sense so far.
Then the next measure is F♯-A-E-B followed by F♯-A-E♭-B , not so clear. According to this page the second measure was analyzed as B7/F♯, i.e. dominant seventh chord, second inversion, with suspended 4th. Which does make sense to me now -- if we write the notes as F♯-A-E-B going to F♯-A-D♯-B .
My question is -- if this harmonic interpretation is correct of course -- why is it notated with E♭ , instead of D♯ ?
According to my youtube-based music theory education, which I would welcome to be corrected on, B to D♯ is a major third and B to E♭ is a diminished fourth, which are enharmonic equivalents but different interval numbers and therefore different meaning. (And maybe even different pitches if not on a well-tempered tuning)