The B major seventh chord is a 4-note chord consisting of the notes B, D#, F# and A#.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Bmaj7.
Chords that are a superset of Bmaj7. The chords include more notes but always B, D#, F# and A#.
Chords that are a subset of Bmaj7. These contain less notes but all of them are included in Bmaj7.
In case you prefer a non-interactive variant of the chord chart, we've embedded a PNG image below that shows the notes for the the Bmaj7 chord. Feel free to save or share the image as needed.