Collisions...

how can I make so that an object A does not collide with object B but collides with everything else?

Would it be an acceptable solution to make object B collision-insensitive, or does object B need to collide with other things in some cases?

object B needs to be able to collide with everything else

I don’t think you can do this with bullet (at least, not the blender implementation)- they focused on physical realism, but there’s almost no character physics, or things like selective collision (stuff that doesn’t happen in real life, but is very useful in approximations of it)