How much is an airliner's drag coefficient?
Is Cd measured at zero lift AoA or when the plane is flying in a straight line?

Both.
$C_D$ is a function, not a number.
The &quot;numbers&quot; are the partial derivatives: $C_{D_\alpha}$ measures how $C_D$ changes w.r.t. the angle of attack $\alpha$, for example.