Chords in the Key of Bb
A song in Bb is built from Bb, C, D, Eb, F, G, A. Those seven notes give you seven chords, and every scale further down is made from the same notes, so all of it fits over the progression.
The seven chords in the key of Bb
One chord built on each note of the Bb major scale. A progression using only these is diatonic, which is what makes the scales below fit it without adjustment.
Scales that fit the key of Bb
The parent scale. Its seven notes are the key, and every chord in the key is built from them.
The relative minor. Identical notes to Bb major, started in a different place, so it fits the same chords with a darker centre.
The major scale with the two half-step notes removed, which is why it is hard to play a wrong note with it.
The same five notes as the major pentatonic, rooted on the relative minor. The default scale for rock and blues soloing in this key.
The minor pentatonic with a chromatic passing note added. Pass through that note rather than landing on it.
The seven modes of Bb
Each of these uses the same notes as Bb major, started on a different degree. They matter when a progression sits on one chord long enough for its own character to come through.
Which one should you actually use?
Start with the G minor pentatonic if you are soloing over a rock or blues progression, and the Bb major pentatonic if the song feels bright. Both are five notes drawn from the key, so neither will fight the chords while you find the phrases you want.