RE: A quick Google search holds all the answer.

Hahah... In this case, we all will assume you've watched game of thrones. Haha.

About using Capcut, I'm guessing that would work if the reaction video would be recorded and manually aligned with the video I'm reacting to. Capcut could be good for it if videos can be exported in HD from the App.

I'm still digging to see if it's possible to record both the reaction and the video I'm reacting to on real-time, as I'm doing it.