When you turn off the "Influence" of the "Copy Rotation Constraint" added last (before 19:32 in the video) , for "Local Y and Z", does this mean that the preceding two "Copy Rotation Constraints" still influence the bone in "Local Y and Z" or does the "Copy Rotation Constraint" added last overwrite everything if its "Influence" is set to "1":
PS1: By copying the "Copy Rotation Constraints" from "MCH_fthrFlight_INT.000" to "MCH_fthrFlight_INT.001" you also copied the different "Influence Setting" for the second "Copy Rotation Constraint" which copies the rotation from "CTRL_fthrFlight.003".
PS2: I've just seen that Blender 2.90 offers a "Mix Mode Setting" for the "Copy Rotation Constraint" which is set to "Replace" per default:
