I have two compositions/animations, A and B.
I play the "A".
How can I, at some point, play the "B" part with a keystroke? For instance, Left Arrow.
Thanks in advance.
There is no such thing. AE is not a tool for producting interactive content. Keystrokes cannot be mapped to expressions.
You can do this in Adobe Character Animator. You could import your AE animations into CA and use Midi device or keystrokes to trigger the animation.