Place it right on the cut to the unknown person, then follow it with the Bruh sound for a second reaction, or switch to Emotional Damage when the reveal turns awkward. Keep this clip first so the question sets up the next reaction.
About this sound
This two-second voice clip has MontanaBlack88 asking Wer ist das denn, meaning Who is that then, in a tone that mixes confusion with a small laugh. It gives a quick hit of comic disbelief, like catching someone who does not belong in the scene. The line comes from the German streamer and spread through German meme edits and reaction clips.