I'm trying to learn improvisation. Are the chords in a saxophone jazz piece supposed to be played in the key of the saxophone or not? This might be a stupid question, but


I'm trying to learn how to read chords and improvising, and I play an Alto Saxophone, however, I can't find anything online to help me even read the chord. I understand that the chord would be, in the most basic sense, arpeggiated, but lets say it says a Bbmaj7.
On an alto saxophone, is that first note in the chord a Bb or actually played at G.
The notes on Bbmaj7 is Bb D F A
Do I play as written Bb D F A? or the transposed G B D F#?
In other words do Bb if it says Bb or do I play G if it says Bb????
I DONT GET IT!!!