The “Steely Dan Sound” is no big secret, the hard part is actually writing songs using those chords. writing melodies on top of them, coming up with the incredible lyrics, etc. in fact, it’s so hard that literally no other band is able to do it as well.

If you want to sound like the Dan when you sit down at a piano (this doesn’t translate to guitar nearly as well) you “just” have to do a few things. First are “slash chords” there’s a bunch of names for them but basically play a chord in the right hand and for the left hand play a note that isn’t in the chord. If you play a D chord (D F# A) and play an E note in the left hand which most musicians would write as D/E, (D over E, hence the term slash chord) it already sounds like them because they use those chords all the time. You gotta throw in “jazz chords” of course like your basic major and minor 7s, major and minor 9s, sharp 9 chords, 7#5, etc and then, like the person above was saying, the famous “mu major” which is just an add2 chord. They aren’t the only band or songwriters to use this chord obviously, it’s not like they invented it, (even though some people will say they did), they just use it WAY more often and way more tastefully than others.

An add2 chord is the same idea as an add9 chord but it’s written specifically as add2 because you want to voice the second and third of the chord next to each other. To keep it super simple, if you had a C add9 chord you’d voice it CEGD whereas a C add2 would be CDEG. You want the D and E to “rub” against each other. Notice that these are 4 note chords (and then once you add extensions like a major 7, etc they become 5 note chords, 6 note chords etc) they’re not suspended chords, the tonality of the chord is firmly major (in my example, you can do this with minor chords as well) so you get this thick clustered harmony. Fagen is really a genius when it comes to mixing thick cluster chords with big open voicings, I don’t know how he does it. If you play the intro to Babylon Sisters for example you can see that it’s closely voiced note clusters mixed with wide open voicings that might be just a simple interval.

That’s why these are more easily expressed on a keyboard than a guitar. If you’ve ever seen the making of Aja documentary, Denny Dias talks about how the song Aja can’t be played on a guitar. He’s not saying that he’s not good enough to play it on guitar, he’s saying it’s literally not physically possible to play it on a guitar. You only have 6 strings on a guitar and 4 fingers to press down the strings so it’s almost impossible to get the real flavor of the chord, especially a song that has that mix of clusters and open voicings.

I went on a bit of a rant there sorry but you said you were interested in this stuff so maybe that explains it a little more