The B♭ Dominant 13th chord and everything around it — the notes and how they are built, the shape on piano and guitar, the sound, and the keys it belongs to.
B♭13 is Bb, D, F, Ab, C, G — the 1, 3, 5, b7, 9 and 13 above the root B♭.
Root notes are red. There is no standard open shape for this chord — any group of these notes on the neck sounds the chord.
Every position of the chord tones on the ukulele neck — any group that sounds only Bb, D, F, Ab, C, G plays the chord.
Work out the chords of a whole key with the Scale & Chord Finder, name a chord from its notes with the Chord Identifier, or browse every chord tool. The theory behind how chords are built is in the free Cloud Atelier music guides.