@jochen80 Welcome to the forum :slightly_smiling_face:
From the rulebook, page 14:
"Any number of defending fighters based in a territory that is about to be strategically bombed can be committed to participate in the defense of the industrial complex as interceptors, whether or not there are attacking fighter escorts. "
What has been bolded by me gives the answer to your questions. When defending air units must return to their ‘original territory’ that is the ‘territory that is about to be strategically bombed’, too, if possible.
HTH :slightly_smiling_face: