Does anyone here know about music theory?

In western music, yes. An octave contains the same intervals for every instrument.

Some instruments are stuck in "keys" so they can't play every note but everything they play sounds good together. Harmonicas for example.

I don’t think so. I know the harp and piano are the same but the clarinet and flute I think are not the same as the harp & piano.

Don't B flat.
Don't B sharp.
Just B natural.
😋

Reminds me of this song. Especially the part after the 2 minute mark.

E flat

A step above d flat is E flat/D sharp. E flat and D sharp are enharmonic meaning different notion but the same tone and note, if you mean an actual full step. If you mean the next note in sequence that being a half step then it's D


Flats and sharps are half steps between notes. Which are normally full steps except between B and C and E and F.. So for example E sharp is the same as an F note and F flat is the same as an E note. Same for B and C notes. For the rest of the scale a sharp of one note is the same as the flat of the next note technically. So A# and Bf are the same note.